Most reef problems come down to something that changed: a parameter drifting, a piece of equipment failing, a new addition, or a change in feeding or maintenance. Check temperature and salinity first, then test alkalinity, nitrate and phosphate, then look back at what you did in the last two weeks.
If something is in distress, act first
If fish are gasping at the surface or several corals have closed up at once, don’t wait to diagnose. Check the heater and the temperature. Make sure the return pump and wavemakers are running and the surface is moving. Have matched saltwater ready, because a water change is the safest general first aid for an unknown problem.
Then work through the checks below.
The checks, in order
Start with the quickest checks and the most common causes. Most problems show up in the first three.
- Temperature. A stuck heater or a hot day can push a tank past 28 °C. Check with a second thermometer.
- Salinity. A failed top-off or a manual top-up with saltwater can move it fast. Aim for about 35 ppt. See reef salinity.
- Equipment. Is everything running? Listen for a pump that has stopped or slowed, and check the skimmer and lights.
- Alkalinity. Test it, and compare with your last few readings. See alkalinity.
- Nitrate and phosphate. Low or high. See nitrate and phosphate.
- Calcium and magnesium. Usually slower to cause trouble, but worth a check.
- What changed? New fish, corals, food, salt, light settings, a moved rock or a dose that stopped.
Common symptoms and where to start
A starting point for each, not a diagnosis.
| What you see | Check first | Read next |
|---|---|---|
| Green hair algae spreading | Nitrate, phosphate, feeding, new additions | Hair algae |
| Corals closed or not opening | Temperature, salinity, alkalinity, recent changes | Water parameters |
| Corals pale or bleached | Temperature, light changes, nitrate and phosphate near zero | Nitrate |
| Alkalinity keeps falling | Coral growth, dosing pump, test kit | Alkalinity dropping |
| Nitrate keeps rising | Feeding, filter socks, skimmer | High nitrate |
| White crust on heaters or pumps | Alkalinity and pH too high | Alkalinity |
| Cloudy water after a water change | New saltwater mixed too hot or too long | How to do a water change |
| Salinity creeping up | Top-off not running | Reef salinity |
| Red or purple slime on the sand | Flow, nitrate and phosphate | Cyanobacteria |
| Brown snotty strands with bubbles | Nitrate and phosphate near zero | Dinoflagellates |
| Brown dust in a new tank | RODI water, silicate | Diatoms |
| Tank over 28 °C in summer | Heater, room, lights | Temperature |
Your records are the best diagnostic tool
When something goes wrong, the most useful question is what’s different from two weeks ago. Without notes, you’re relying on memory. With a dated log of tests, water changes and maintenance, you can often see the cause straight away: an alkalinity reading that slid, a water change that was skipped, a filter sock that was left for a fortnight.
That’s the job the Companion is built for. Log your tests and tick off your maintenance, and the history is there when you need it.
Change one thing at a time
Once you have a likely cause, make one change and give it time to show. If you change the feeding, the filtration and the lighting all at once, you won’t know which one helped, and you risk a second problem. Note what you changed and when, then retest.
If you can’t find a cause, take your test results to an experienced local shop or reef club. A log of readings over several weeks makes their advice far more useful than a single test.
Common questions
What should I test first when something goes wrong?
Temperature and salinity, then alkalinity, nitrate and phosphate. They cover the most common causes.
Should I do a water change if I don’t know what’s wrong?
A water change with matched saltwater is a safe general first step for many problems. Match temperature and salinity carefully.
How do I stop the same problem coming back?
Keep notes. Most repeat problems come from a routine that slipped or a change that wasn’t noticed.
Sources and limits
Last checked 12 October 2026. This guide brings together published advice and simple maths. It can’t see your tank. If livestock is in distress, act first, then ask an experienced local shop or club.
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Bulk Reef Supply: getting rid of green hair algae
- Bulk Reef Supply: 5 ways to lower nitrates
- Red Sea: how to mix salt