My Reef Tank Is Crashing. What to Check First

Stop dosing and feeding first — you can't diagnose a tank you're still changing. Then check equipment that fails silently (heater, auto top-off, pumps), test salinity, temperature, alkalinity, and ammonia, and ask what changed in the last two weeks. Most crashes trace back to a recent change.

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.

The Field Guide is written by AlongSide.Guide. AlongSide connects you with Guides who get paid to work alongside you, and some links on this page go to it.

Questions people ask

What's the first thing to do when a reef tank is crashing?

Stop all dosing, additives, and feeding immediately — you can't diagnose a system you're still changing.

What should I test when a reef tank crashes?

The core four: salinity, temperature, alkalinity, and ammonia. Write down the numbers with the time you tested.

What usually causes a reef tank crash?

A recent change — new livestock, new rock, a new salt brand, a cleaning session, a power outage, or a new light schedule.

Is a water change safe during a reef tank crash?

Only if the new water matches temperature and salinity exactly. A mismatched water change is a second shock on top of the first.

Your three options

Do it alone

Stopping dosing, checking equipment, and testing the core four numbers is something you can do immediately, yourself.

With a Guide

Someone who's seen this exact crash can often diagnose it from your test log and photos in minutes — before a forum thread even gets going.

Hire it out

A service call might not arrive until after the tank is already lost. Crashes move faster than most in-person scheduling.