What is dark start cycling?

Dark start (also called dark period cycling) is a method designed specifically for planted aquariums. You fill the tank, plant it densely, and keep the lights completely off for three to four weeks. Without light, algae cannot photosynthesise and cannot establish — but your plants can still develop root systems and bacteria colonise the filter media. By the time you turn the lights on, the biological cycle is already running and plants are established enough to outcompete algae from the very first day.

The ammonia source in a dark start comes from decomposing plant matter and the off-gassing of nutrient-rich substrates (ADA Aqua Soil, Tropica Soil, and similar active substrates release ammonia as they mature). No external ammonia dosing is needed, though a daily pinch of fish food supplements leaner substrates.

When to choose dark start

• You are setting up a planted freshwater tank (not suitable for fish-only or marine setups)
• You want to avoid the algae outbreak that typically hits newly cycled planted tanks
• You are using a nutrient-rich active substrate, or can plant densely with fast-growing stems
• You are not in a rush — the dark period takes at minimum 3 weeks

What you’ll need

• Fast-growing stem plants for initial plant mass: Hygrophila, Bacopa, Vallisneria, Rotala — species that root quickly and absorb nutrients rapidly
• A nutrient-rich substrate (ADA Aqua Soil, Tropica Substrate, Seachem Flourite) — or inert substrate plus a daily pinch of fish food
• An all-in-one liquid fertiliser (Tropica Specialised, Easy Life ProFito)
• Liquid test kit for ammonia, nitrite, and nitrate
• A running filter and heater (22–26 °C)

Step by step

Step 1 — Fill tank, add substrate and plants

Add substrate before filling if using active substrate — it must be submerged correctly. Plant densely with fast-growing stems. Hardscape (rocks, driftwood) can go in now. Do not add fish or snails yet.

Hint: Do not add hardscape or fish yet. Dense planting helps absorb nutrients during cycling.

Step 2 — Keep lights off for the first 3–4 weeks

Cover the tank or set the light timer to 0 hours. Total darkness is the goal — algae cannot establish without light. Plants can still develop roots and absorb nutrients through their leaves in ambient room light, which is too dim for algae.

Hint: No light prevents algae while plants root and bacteria colonise.

Step 3 — Dose fertiliser lightly from day 1

Even in the dark, plants need nutrients to develop roots and stay alive. Add half the manufacturer’s recommended dose of all-in-one liquid fertiliser once or twice per week from day one.

Hint: Plants need nutrients even in the dark. Use a low dose of all-in-one liquid fertiliser.

Step 4 — Test parameters weekly — wait for ammonia and nitrite to clear

Test ammonia, nitrite, and nitrate once a week. Expect ammonia to appear in week 1–2 (from substrate or fish food), then drop as nitrite rises, then nitrite drops and nitrate rises. Active substrates can push ammonia to 4–6 ppm in the first week — this is normal.

Hint: Decomposing plant matter and substrate provide the ammonia source.

Step 5 — Turn on lights gradually and confirm cycle complete

Once ammonia and nitrite both read 0 and nitrate has risen, the cycle is complete. Introduce light gradually: start at 4 hours per day and increase by 1 hour per day up to your target photoperiod (typically 8–10 hours). A sudden jump to full photoperiod after total darkness triggers algae blooms.

Hint: Start with 4–6 hours per day and ramp up over a week to avoid algae shock.

Common mistakes

Introducing light too quickly. After weeks of darkness, even one day at full photoperiod can seed a cyanobacteria or green algae outbreak. Ramp up over 7–10 days to let plants get ahead of algae.

Using only slow-rooting plants. Crypts and anubias are beautiful but slow to establish. For a dark start, you need fast-growing stems taking up nutrients from day one. Add them as the bulk of your initial planting; you can remove some once the tank is established.

Expecting inert substrate alone to cycle. Plain gravel and sand provide almost no ammonia. Add a pinch of fish food daily if not using an active substrate, or the bacteria will have nothing to eat.

How long to expect

Phase	What’s happening	Typical duration
Weeks 1–2	Substrate off-gasses ammonia, plants root	2 weeks
Weeks 2–3	Ammonia drops, nitrite rises	1 week
Weeks 3–4	Nitrite drops, nitrate rises	1 week
Week 4–5	Confirm complete, ramp lights	1 week

Active substrates (ADA Aqua Soil) release a large ammonia pulse in the first two weeks — readings of 2–6 ppm are normal. Inert substrates with daily fish food produce lower, slower ammonia curves and may extend the timeline.

AquaKeepers has a built-in dark start guide that tracks each phase — log your weekly readings and the app shows when you are ready to turn the lights on.