The pH Crash

What’s Happening

pH crashes are usually caused by low KH (carbonate hardness), which acts as a buffer. Without KH, CO₂ build-up from fish respiration acidifies the water rapidly overnight.

Steps to Fix It

1. Do a small 20% water change only. A large water change could rapidly raise pH and shock fish — worse than the low pH.
2. Test your tap water KH. If KH is below 2 dKH, your source water can’t buffer pH — you need to address this long-term.
3. Add crushed coral to your filter (about 1 cup per 50 gallons). It dissolves slowly and raises KH and pH gently. Alternatively, use a commercial KH buffer product per its dosage instructions. If you keep CO₂-sensitive invertebrates, consult their care sheet first.
4. Retest pH in 24 hours and log the result.

Prevention

• Maintain KH above 3–4 dKH as a pH buffer.
• If you use CO₂ injection for plants, ensure your overnight pH drop is less than 1 unit.

Track your pH readings and get step-by-step crisis alerts in the AquaKeepers app.