Why an Auto Top-Off System Is the Best $50 You'll Spend

The problem ATOs solve

Water evaporates continuously from any open-top tank or sump, but the salt and minerals don't — every drop of evaporation concentrates whatever's left behind, gradually raising salinity (and effectively concentrating other parameters too) until you top off with fresh water.

Why manual top-off isn't good enough

Evaporation rate isn't constant — it changes with room temperature, humidity, and season. Topping off "when you remember" or on a fixed schedule means salinity drifts up between top-offs and swings back down right after, a sawtooth pattern that's exactly the kind of instability sensitive coral dislike.

What a basic ATO setup involves

A float switch or optical sensor in your sump triggers a small pump to add fresh RO/DI water automatically, holding water level (and therefore salinity) essentially constant. Basic single-pump setups cost well under $100 and are one of the highest-impact, lowest-cost upgrades available for stability.

The one real risk to manage

A stuck-open float switch can overfill or, more dangerously, keep adding water indefinitely if the reservoir runs dry mid-failure — pairing your ATO with a simple backup low-water shutoff or reservoir-empty sensor closes that failure mode cheaply.