Why Saltwater Tanks Lose Water So Quickly

Introduction

Setting up your first saltwater aquarium is an incredibly exciting milestone. You have likely spent weeks researching filtration, choosing the perfect tank placement, selecting vibrant live rock, and dreaming of the colorful fish and corals you will eventually bring home. But after only a few days of running your new system, you are likely to experience a sudden moment of alarm. You look at the water line in your tank or sump, and you realize it has dropped by a noticeable margin—perhaps half an inch, an inch, or even more.

For an absolute beginner, this rapid loss of water can trigger immediate panic. Your mind naturally jumps to the worst-case scenario: a slow leak somewhere in the silicone seams, a cracked bulkhead in your overflow plumbing, or a failing seal in your filtration equipment. You might find yourself down on your hands and knees with a flashlight, checking every corner of your stand for telltale signs of moisture.

In the vast majority of cases, however, your tank is not leaking at all. What you are witnessing is the powerful, constant, and surprisingly rapid process of evaporation. While evaporation occurs in all aquariums, saltwater systems are engineered in a way that dramatically accelerates this physical process.

Understanding why saltwater tanks lose water so quickly, how this loss impacts the delicate chemistry of your marine environment, and how to manage it safely is one of the most fundamental skills you must master as a marine aquarist. Unlike freshwater aquariums, where water loss is mostly a cosmetic issue that you address during weekly maintenance, evaporation in a saltwater aquarium is a daily chemical event. If ignored, it can rapidly destabilize your salinity, stress your livestock, and lead to catastrophic tank failure.

This comprehensive guide will walk you through the differences between evaporation and leaks, the underlying physics of water loss in marine systems, the biological dangers of rising salinity, and the tools and strategies you need to keep your tank stable, healthy, and thriving.

Evaporation vs. Leaks: How to Tell the Difference

When you first notice that your tank is losing water, it is absolutely essential to rule out a structural leak before assuming it is simply evaporation. Finding a slow leak early can save your floors, your equipment, and the lives of your aquatic inhabitants. Fortunately, there are several reliable ways to distinguish between normal evaporation and a dangerous leak.

The Return Pump Chamber Test

If your saltwater tank is equipped with a sump—a secondary filtration tank located inside the cabinet beneath your main display—you will notice a unique physical phenomenon. The water level in your main display tank will remain completely constant, while the water level in the sump will drop.

This happens because of the way overflows and return pumps function. The return pump pushes water from the sump up into the display tank. Excess water in the display tank then spills over the overflow weir and drains back down into the sump. Because the height of the overflow weir is fixed, the display tank will always remain filled to that exact level. Any water lost from the system through evaporation will show up exclusively in the sump, specifically in the final chamber where the return pump is located.

If you notice the water level in your display tank dropping while your return pump is running, this indicates a serious problem. It means either your return pump has failed, your overflow drain is blocked, or you have a major leak in the display tank itself.

The Salt Creep Clue

One of the most unique aspects of saltwater aquariums is “salt creep.” When saltwater splashes, drips, or slowly leaks, the water evaporates on the dry external surfaces, leaving behind a crusty, white, powdery deposit of pure salt.

If your aquarium has a physical leak, you will almost always find a heavy accumulation of salt creep directly around the source of the moisture. Inspect the following areas weekly for salt creep:

The bulkheads and plumbing joints beneath the tank.
The seams where the glass pane meets the plastic trim or silicone joints.
The area surrounding your protein skimmer, media reactors, and return pump connections.
The edges of your sump.

If you find clean, dry glass and plumbing with no white salt crusting, your water loss is almost certainly due to normal evaporation. If you find a damp patch surrounded by a thick ring of white salt, you have found a leak that must be addressed immediately.

The Paper Towel and Cardboard Test

Slow leaks can sometimes be incredibly difficult to spot with the naked eye, especially if the water is trickling down the back of a black stand or escaping onto a carpeted floor. To verify if moisture is escaping, place pieces of dry cardboard or folded paper towels directly beneath and behind the stand, plumbing joints, and sumps.

Leave these in place for 24 hours. Because dry cardboard and paper towels absorb moisture instantly, even a single drop of leaking water will leave a visible, warped wet spot or a salt stain, allowing you to trace the leak back to its origin.

The Chemistry and Physics of Saltwater Evaporation

Once you have confirmed that your aquarium is dry on the outside, you can turn your attention to the science of evaporation. To manage your tank effectively, you must understand the physical and chemical factors that drive water loss.

Does Saltwater Evaporate Faster Than Freshwater?

There is a common misconception among beginners that saltwater naturally evaporates faster than freshwater. In terms of pure chemistry, the exact opposite is true.

According to Raoult’s Law in physics, when a solute (such as salt) is dissolved in a solvent (such as water), the vapor pressure of the solvent decreases. In simple terms, the dissolved salt molecules occupy space at the surface of the liquid, physically blocking some of the water molecules from escaping into the air. Under identical, static laboratory conditions (same temperature, same surface area, no airflow), freshwater will actually evaporate slightly faster than saltwater.

Why, then, do saltwater tanks in the home seem to lose water at such an alarming rate? The answer lies not in the water chemistry itself, but in the mechanical design, high flow rates, and environmental conditions of marine aquariums. Saltwater tanks require massive amounts of surface agitation, open-topped designs, heat-generating lighting, and active air-injection filtration systems—all of which dramatically accelerate the rate of evaporation far beyond what is typical in a standard freshwater setup.

The Phase Transition of Water

Evaporation is a phase transition where liquid water turns into water vapor (a gas). This occurs when individual water molecules near the surface absorb enough thermal energy (heat) from their surroundings to overcome the intermolecular forces holding them in liquid form.

Once a water molecule escapes into the air, it becomes humidity. If the air above the water is dry and constantly moving, more water molecules can escape. If the air is stagnant and saturated with moisture, evaporation slows down. In the home aquarium, we are constantly manipulating these variables, often without realizing it, to create a perfect storm for rapid water loss.

The Critical Danger: The “Salt Stays Behind” Rule

To understand why evaporation is such a critical concern for marine aquarists, you must internalize the golden rule of aquarium chemistry: water evaporates, but salt does not.

When liquid water transitions into gas and escapes your tank, it leaves every single dissolved salt ion, mineral, and metal behind in the remaining liquid. The sodium, chloride, magnesium, calcium, and carbonates that make up your salinity do not change in quantity. However, because the total volume of water holding these minerals has decreased, the concentration of salt increases.

Understanding the Mathematical Impact

Let us look at a simple hypothetical example to visualize this chemical shift:

You have a 30-gallon aquarium filled with saltwater at a healthy, natural ocean salinity of 35 parts per thousand (ppt). This means you have a specific ratio of salt dissolved in 30 gallons of water.
Over the course of a hot, dry week, 3 gallons of water evaporate from your tank. You now have only 27 gallons of water remaining.
Because all of the salt stayed behind, that same total mass of salt is now packed into a smaller volume of water. Your salinity will rise from 35 ppt to approximately 39 ppt.

A salinity of 39 ppt is highly stressful for almost all marine organisms, and if the water level continues to drop, the salinity will quickly reach levels that are fatal to your tank’s inhabitants.

The Biological Toll of Salinity Fluctuations

Why are marine organisms so sensitive to these shifts? The answer lies in their physiology.

Osmoregulation in Marine Fish

Bony marine fish are “osmoregulators.” The internal salinity of a marine fish’s blood and bodily fluids is naturally much lower than the salinity of the surrounding ocean (typically about one-third of the concentration). Because of this difference, water is constantly being drawn out of the fish’s body and into the saltier ocean water through their semi-permeable skin and gills via osmosis.

To prevent dehydration, marine fish must constantly drink large quantities of saltwater, extract the pure water, and use specialized, energy-intensive cells in their gills to pump the excess salt back out into the aquarium.

When your salinity rises due to evaporation, the osmotic pressure increases. Water is pulled out of the fish’s body even faster, forcing their kidneys and gills to work twice as hard to maintain balance. This expends massive amounts of energy, leaving the fish exhausted, lowering their immune systems, and making them highly susceptible to deadly parasites like Marine Ich (Cryptocaryon irritans) or Velvet (Amyloodinium ocellatum).

Osmoconformers: Corals and Invertebrates

While fish can actively fight salinity changes, corals and invertebrates (such as snails, crabs, shrimp, starfish, and sea urchins) cannot. They are “osmoconformers.”

The salinity of an invertebrate’s internal fluids is completely identical to and dictated by the salinity of the water around them. If the salinity of your tank rises, water immediately rushes out of the invertebrate’s cells to balance the concentration.

For a coral, this osmotic shock causes immediate tissue retraction, stops photosynthesis in their symbiotic algae (zooxanthellae), and can lead to rapid tissue necrosis (RTN), where the coral’s flesh literally sloughs off the skeleton in a matter of hours. For sensitive invertebrates like starfish or sea urchins, a sudden shift in salinity will cause them to lose limbs, drop spines, and quickly perish.

NEVER allow your aquarium’s salinity to fluctuate by more than 0.5 ppt (or 0.0005 Specific Gravity) in a single day. Maintaining a rock-solid, stable salinity is the single most important factor in keeping corals and marine invertebrates alive.

Five Key Drivers of Saltwater Tank Evaporation

To manage evaporation, you must identify the mechanical and environmental factors in your setup that are driving it. Here are the five primary reasons why saltwater tanks lose water so quickly.

1. High-Flow Wavemakers and Surface Agitation

If you look at a healthy coral reef, you will see constant, violent wave action. Marine organisms have evolved to thrive in high-energy water flow, which delivers nutrients, removes waste products, and keeps oxygen levels saturated. In the home aquarium, we replicate this using high-power wavemakers, powerheads, and return nozzles.

To prevent film buildup and ensure proper gas exchange, aquarists deliberately aim these pumps at the surface of the water, creating vigorous ripples, waves, and surface agitation.

While this surface movement is excellent for your tank’s health, it drastically increases evaporation. The constant rippling expands the actual surface area of the water exposed to the air. Instead of a flat, stagnant sheet of water, you have a dynamic, three-dimensional surface of moving water. This increased surface area gives water molecules many more opportunities to escape into the atmosphere.

2. Sump Systems and Open Overflows

Most mid-to-large-sized saltwater aquariums utilize a sump system. Water flows out of the main tank through an overflow box, rushes down plumbing lines, drains into the sump in the cabinet below, passes through various filtration chambers, and is pumped back up to the display tank.

This continuous cycle dramatically accelerates water loss in several ways:

Increased Surface Area: Adding a sump essentially adds a second, open body of water to your system. A 75-gallon tank with a 20-gallon sump has roughly 30% more surface area exposed to the air than a tank without a sump.
Agitation and Splashing: As water cascades over the overflow teeth, crashes down the drain lines, and spills over the internal baffles of the sump, it undergoes violent agitation. This turbulent water-to-air contact allows water vapor to escape continuously.
Filter Socks and Media: Water trickling through porous filter socks or over mechanical fleece rollers creates a large surface area of thin, moving water films, which evaporate at an accelerated rate.

3. Open-Top Designs vs. Solid Lids

In the freshwater hobby, almost every tank comes equipped with a solid plastic hood or glass canopy. These lids act as barriers to evaporation; water evaporates, condenses on the underside of the lid, and drips right back into the aquarium.

In contrast, the vast majority of modern saltwater tanks are completely open-topped or utilize loose mesh screen lids. There are two primary reasons for this design choice:

Gas Exchange and pH Stability: Saltwater tanks require high oxygen levels and the constant release of dissolved carbon dioxide ($CO_2$). If $CO_2$ builds up in the water, it forms carbonic acid, which causes your pH to drop to dangerous levels. An open top allows $CO_2$ to escape freely into the room.
High-Intensity Lighting: Marine corals require extremely powerful specialized LED, T5, or metal halide lighting to photosynthesize. Solid glass lids reflect a significant portion of this light energy, reducing the PAR (Photosynthetically Active Radiation) reaching your corals. Furthermore, glass lids quickly become coated in dried salt creep, which acts as a physical block to light penetration.

While open-top tanks are fantastic for coral growth and pH stability, they allow water vapor to escape directly into your home without any condensation barrier.

4. Protein Skimmers and Air Injection

The protein skimmer is the heart of a saltwater tank’s filtration system. It works by using a specialized pump equipped with a needle wheel impeller to draw in massive volumes of air and water, blending them into a dense column of micro-bubbles inside a acrylic chamber.

As these bubbles rise through the chamber, organic waste adheres to their surfaces, forming a thick foam that spills over into a collection cup.

This process is essentially an evaporation machine. You are injecting millions of tiny air bubbles directly into the water column. Each bubble represents a tiny pocket of dry air that immediately saturates with water vapor as it rises. When the bubbles pop at the top of the neck, this highly humid air is expelled out of the skimmer lid and into the room. A well-performing protein skimmer can easily account for a significant portion of your daily water loss.

5. Temperature Differentials and Indoor Climate Control

To keep tropical marine life healthy, you must maintain a stable water temperature, typically between 76°F and 80°F.

In most homes, the ambient air temperature is kept significantly cooler than the aquarium water, especially during the winter or in air-conditioned summer months (usually between 68°F and 72°F).

This temperature difference creates a vapor pressure gradient. Warm water molecules naturally want to migrate toward the cooler, drier air. If your home has low humidity (which is common when running central heating or air conditioning), the air acts like a dry sponge, actively pulling moisture out of the warm aquarium surface.

The Auto Top-Off (ATO): Your Tank’s Best Friend

Now that we know why saltwater tanks lose water so quickly, we must address how to manage this loss. In the early days of the hobby, aquarists managed evaporation manually: they would look at the tank every morning, carry a jug of freshwater to the tank, and pour it in until the water reached a marked line on the glass.

While this manual method works in a pinch, it is highly detrimental to the stability of a marine system.

If you top off your tank manually once a day, your salinity behaves like a roller coaster. Throughout the day, water evaporates, and the salinity slowly creeps upward. When you suddenly dump in a large volume of freshwater, the salinity plummets back down in a matter of seconds. This constant cycle of rising and falling salinity keeps your fish and corals in a state of perpetual osmotic stress.

The solution to this problem is an Auto Top-Off (ATO) system. An ATO is an automated device that monitors the water level of your aquarium and automatically replenishes evaporated water in tiny, frequent increments, maintaining a completely stable salinity 24 hours a day.

graph TD
    A[Sump Return Chamber] -->|Water Evaporates| B(Water Level Drops)
    B --> C{ATO Sensor Detects Drop}
    C -->|Yes| D[ATO Controller Activates Pump]
    D --> E[RO/DI Water Pumped from Reservoir]
    E --> F[Water Level Restored / Salinity Stabilized]
    C -->|No| G[System Idle]

How an ATO System Works

A standard ATO setup consists of four main components:

A Sensor: This is placed in the return pump chamber of your sump (or directly in the display tank if you do not have a sump). It detects when the water level drops by as little as a few millimeters. Common sensors include mechanical float switches, optical infrared sensors, or temperature-differential sensors.
A Controller: The brain of the unit. It receives the signal from the sensor and decides when to turn the pump on and off. Most modern controllers feature built-in safety timers to prevent over-filling.
A Feed Pump: A small, low-voltage submersible pump placed inside your freshwater reservoir.
A Freshwater Reservoir: A dedicated bucket, acrylic container, or glass tank filled with pure freshwater.

When the sensor detects a drop in the water level, it signals the controller, which activates the pump. The pump pushes freshwater into the sump until the sensor is submerged again, turning the pump off. Because this occurs multiple times an hour, your salinity remains perfectly stable.

Crucial Safety Protocols for ATO Systems

An ATO system is one of the most useful pieces of equipment you can buy, but if set up incorrectly, it can cause catastrophic failures. If an ATO sensor sticks in the “on” position, the pump will continuously push freshwater into your tank, diluting the salinity, killing your livestock, and flooding your home.

To prevent this, you must follow these safety-critical guidelines:

NEVER run an ATO feed line directly from your home’s RO/DI filtration unit into your sump. Always feed the water from a standalone, limited-volume reservoir. If you connect your ATO directly to your home’s water supply and the sensor fails open, you will have an infinite supply of freshwater flooding your tank. By using a physical reservoir (such as a 5-gallon bucket), the maximum amount of freshwater that can accidentally enter your system is limited to the volume of that container, which is far less likely to cause a total collapse.
ALWAYS position the end of the ATO output tube higher than the water level in your freshwater reservoir. If the outlet of the tube in your sump is lower than the water level in your reservoir, you will create a natural gravity siphon. Once the pump turns on and starts flowing, the water will continue to flow even after the pump turns off, siphoning the entire contents of the reservoir into your tank.
Keep your sensors clean and free of obstructions. Snails, algae, and salt creep love to cling to ATO sensors. A single small snail crawling onto a mechanical float switch can hold it down, tricking the controller into thinking the water level is low. Clean your sensors with a soft brush and fresh water monthly.
Use dual-sensor systems. Choose an ATO that features a primary optical sensor and a secondary mechanical float switch positioned slightly higher up as a physical backup. If the optical sensor fails, the float switch will physically cut power to the pump before a flood occurs.

What Water to Use: The Golden Rule of Top-Offs

When addressing evaporation, the quality of the water you use to replenish your system is just as important as the mechanics of how you add it.

The Difference Between Water Changes and Top-Offs

As an absolute beginner, you must memorize the difference between these two maintenance tasks:

Water Changes (Saltwater): When you physically extract dirty water from your tank during maintenance, you are removing both water and salt. Therefore, you must replace it with newly mixed saltwater of the exact same salinity and temperature.
Topping Off (Freshwater): When water evaporates from your tank, the salt stays behind. Therefore, you must replace it with pure freshwater.

NEVER top off evaporated water with saltwater. If you use saltwater to replenish your evaporation, you are adding new salt to the salt that remained in the tank. Your salinity will rise continuously with every top-off, quickly reaching levels that are toxic to your marine life.

Why Tap Water is Destructive to Marine Aquariums

Now that you know you must use freshwater for top-offs, you might be tempted to simply fill a jug from your kitchen sink or use bottled spring water. Doing so is one of the most common reasons beginners fail in the saltwater hobby.

Tap water contains a wide variety of dissolved solids, chemicals, and impurities. While municipal water treatment plants make tap water safe for human consumption by adding chlorine and chloramines, these chemicals are highly toxic to aquatic life. Even if you use a liquid dechlorinator, tap water still contains phosphates, nitrates, silicates, copper, lead, and iron.

[Tap Water Top-Off] ---> [Aquarium Evaporation] 
                               |
                               v
                       [Pure H2O Escapes] 
                               |
                               v
               [Toxic Impurities Accumulate & Concentrate] 
                               |
                               v
             [Algae Blooms, Coral Death, Organ Failure]

When you top off your tank with tap water, the pure water evaporates, but all of these impurities—including heavy metals, phosphates, and nitrates—are left behind in the tank. Every time you add tap water, you are adding more impurities, which build up over time. Within a few months, this accumulation will trigger severe hair algae outbreaks, cover your glass in brown diatoms, and poison sensitive corals and invertebrates.

The RO/DI Standard

To ensure the long-term health of your tank, ALWAYS use pure Reverse Osmosis/Deionized (RO/DI) water for all top-offs and salt mixes.

An RO/DI system is a multi-stage filtration unit that forces tap water through sediment filters, carbon blocks, a semi-permeable reverse osmosis membrane, and a deionization resin. This process strips away 99.9% of all dissolved solids, leaving behind pure $H_2O$ with a Total Dissolved Solids (TDS) reading of exactly 0 parts per million (ppm). By starting with 0 TDS water, you have complete control over the chemistry of your aquarium, ensuring that no unwanted minerals or toxins accumulate in your system over time.

You can purchase your own RO/DI unit to install under a sink, or you can purchase pre-filtered RO/DI water from a local fish store.

Practical Tips for Managing Evaporation

Managing your tank’s water loss does not have to be difficult. By implementing these practical habits, you can keep your system stable and minimize the physical labor of maintaining it.

Mark Your Baseline

If you do not have an ATO system yet and are topping off your tank manually, you must establish a clear visual reference point.

Fill your tank and sump to their optimal operating levels.
Ensure your salinity is sitting exactly at your target level (e.g., 35 ppt or 1.026 Specific Gravity).
Use a piece of colored electrical tape or a dry-erase marker to draw a clear line on the glass of your sump (or display tank) at the current water level.
Check this line every morning and evening. Add RO/DI freshwater until the water level returns exactly to the line.

Calculate Your Daily Evaporation Rate

Knowing exactly how much water your tank loses daily is crucial for planning your water storage and preparing for trips. To calculate your rate:

Top off your tank to your baseline mark.
Turn off your ATO system for exactly 24 hours.
After 24 hours, use a measuring cup to add RO/DI water back to the baseline mark, keeping track of how many cups or liters you add.
Turn your ATO system back on.

This number is your daily evaporation rate. A typical open-top 75-gallon saltwater tank can easily lose 0.5 to 1 gallon of water per day, depending on the room’s temperature and humidity.

Plan for Vacations

Your daily evaporation rate dictates how long you can safely leave your tank unattended. If you have a 5-gallon reservoir connected to your ATO and your tank evaporates 1 gallon of water per day, your system can only run safely for 5 days before the reservoir runs dry.

If the reservoir runs dry, the ATO pump will run dry and burn out, the water level in your sump will drop, and your main return pump will begin sucking in air, running dry, and overheating. This can quickly result in a total system crash.

If you plan to go on vacation for a week or more, you must calculate your total water needs and temporarily connect your ATO to a larger reservoir (such as a clean, food-grade 32-gallon trash can filled with RO/DI water) to ensure the system does not run dry while you are away.

Common Evaporation and Salinity Mistakes to Avoid

To ensure your success in keeping marine life, avoid these common pitfalls that beginners often encounter when managing water levels and salinity.

Mistake 1: Topping Off with Saltwater

As discussed, this is the quickest way to spike your salinity to toxic levels. Remember: evaporation = freshwater replacement. Reserve your saltwater exclusively for physical water changes where you are removing old saltwater from the system.

Mistake 2: Using Tap Water or Bottled “Spring” Water

Tap water contains chlorine, chloramine, and heavy metals. Bottled spring water contains added minerals for taste, which are highly detrimental to a marine reef. NEVER wash biological filtration media in tap water either, as the chlorine will instantly kill the beneficial nitrifying bacteria. Only use pure 0 TDS RO/DI water for top-offs.

Mistake 3: Creating a Gravity Siphon on the ATO Line

If the end of your ATO output hose is submerged in your sump water, or if it is positioned lower than the maximum water level of your freshwater reservoir, a siphon will form.

When the ATO pump runs, it fills the line. Once the pump turns off, gravity will continue to draw water out of the higher reservoir and dump it into the lower sump. This will continue until the water levels equalize, flooding your sump with freshwater and severely diluting your salinity.

ALWAYS mount your ATO output line high above the water line of your sump, and ensure it terminates at a point higher than the highest water level in your reservoir.

CORRECT SETUP (No Siphon):
+-----------------+
| Reservoir (High)|          ATO Output
|   [Water]       |========= \ (Air Gap - Above Water Level)
+-----------------+           \ 
                               V
                         +-----------+
                         | Sump      |
                         +-----------+

INCORRECT SETUP (Siphon Hazard):
+-----------------+
| Reservoir (High)|
|   [Water]       |=========
+-----------------+         \ 
                             \====== [Sump Water Level]
                                     (Submerged - Siphon Will Flow!)

Mistake 4: Calibrating Your Refractometer with Pure Water

Many optical refractometers instruct you to calibrate the device to zero using distilled water or RO/DI water.

While this works for simple sugars, it introduces a significant error when measuring saltwater. Refractometers are highly sensitive to the refractive index of different liquids. Calibrating at “0” can cause the device to read 1.026 when your actual tank salinity is only 1.023 or 1.024.

ALWAYS calibrate your refractometer using a dedicated 35 ppt seawater calibration fluid. This ensures the device is calibrated at your exact target measurement, providing absolute accuracy where it matters most.

Mistake 5: Allowing Snails to Block Sensors

Invertebrates like snails love to graze on the algae that grows on ATO sensors. If a heavy turbo snail climbs onto a floating magnetic switch, its weight can hold the switch down, keeping the pump running and flooding the tank. Use sensors that feature protective plastic guards, or select optical sensors that are too small for snails to sit on.

Conclusion

The rapid loss of water in a saltwater tank can be surprising to an absolute beginner, but it is a normal, healthy physical process. The high flow rates, surface agitation, open-topped designs, and active filtration systems that make marine tanks successful are the very same factors that drive high evaporation rates.

By understanding that water evaporates while salt stays behind, you can appreciate the critical importance of daily freshwater top-offs. Whether you choose to top off your tank manually every day or invest in a reliable Auto Top-Off system, maintaining a stable salinity is the baseline of success in the marine hobby.

Always use pure RO/DI water for your top-offs, keep your sensors clean, set up your plumbing with proper siphon breaks, and monitor your salinity weekly with a calibrated refractometer. With these habits established, you will prevent dangerous salinity swings, minimize stress on your fish and corals, and build a stable, thriving reef ecosystem that you can enjoy for years to come.