Emergency playbook

Auto top-off failed: salinity too high or too low

A stuck top-off can drop salinity fast; an empty one raises it slowly over days. Both are fixable if you correct slowly.

Why each step

1. Unplug the top-off pump.

If it is stuck on, every minute adds more fresh water. If it is empty, nothing is lost by unplugging it.

2. Measure salinity with a refractometer calibrated in 35 ppt fluid.

You need the number before you correct anything.

3. Check the sump level against your normal water line.

Water near the rim with low salinity means the top-off stuck on. A low sump with high salinity means it ran dry or failed off (BRS).

4. If salinity is low, stop all top-off and let evaporation raise it, or replace some water with salt water mixed slightly above target.

Our reading: change salinity by no more than about 1 ppt a day. No source we found sets a rate.

5. If salinity is high, top up with RO/DI water in small steps over a day or two.

The same 1 ppt a day reading applies on the way down.

Once the tank is stable, what next?

1. Check the float switch or sensor for snails, algae or salt creep.
2. Retest salinity every day until it is back on target.
3. Watch corals and invertebrates: they react to salinity changes more than fish.

How do I stop it happening again?

• Use a top-off with a second, backup sensor that cuts the pump if the first fails (BRS).
• Keep the reservoir small. If it all went in at once, salinity should not fall below about 32 ppt. Our sum: the reservoir should hold no more than about 9% of your net system volume. BRS keeps most reservoirs at or under 10 US gallons (about 38 litres).
• Never plumb RO/DI straight into the reservoir without a float valve and shut-off (BRS).
• Use only 0 TDS RO/DI water for top-off (BRS).
• Run the top-off for a week before relying on it while you are away.

Last checked 21 October 2026. How we check