My Reef Tank Is Crashing. What to Check First

When a reef crashes, the instinct is to do everything at once. That instinct is usually what finishes the tank. Slow down and work in order.

1. Stop adding things. Pause dosing, new additives, and feeding for now. You can't diagnose a system you're still changing.

2. Check the equipment that fails silently

• Heater stuck on or off: check the thermometer and touch the glass.
• Auto top-off failed or overran: an ATO dumping fresh water crashes salinity, and a dead one lets it climb.
• Return pump, powerheads, skimmer overflowing into the sump.

3. Test the core four. Test salinity, temperature, alkalinity, and ammonia. Write the numbers down with the time.

4. Ask what changed in the last two weeks. New livestock, new rock, new salt brand, a cleaning session, a power outage, a new bulb schedule. Crashes almost always trace back to a change.

5. Water change, carefully. If something's clearly off, a water change helps only if the new water matches temperature and salinity. A mismatched water change is a second shock.

Why this is where an expert pays for themselves. Someone who's seen this exact crash can often tell from your photos and test log in minutes whether you're looking at a salinity swing, an alk crash, or a pest problem. Forums give you ten conflicting answers over two days, and a crashing tank doesn't have two days.