Dive Deeper: Sizing and Choosing an ATO Reservoir
How Big a Reservoir Do You Need?
Size the reservoir to your evaporation rate, not to your tank volume. Measure it: mark your sump water line, wait 24 hours with the ATO off, and top off with a measuring jug. That number is your daily loss.
| Daily evaporation | Recommended reservoir | Refill interval |
|---|
| Under 0.5 gal | 5 gallon | ~10 days |
| 0.5–1 gal | 10 gallon | ~10 days |
| 1–2 gal | 20 gallon | ~10 days |
| Over 2 gal | 20 gallon + external barrel | Weekly |
Target roughly seven to ten days of capacity. Much less and you are back to a daily chore; much more and a single ATO sensor failure can dump a very large volume of fresh water into the system.
Shape Matters More Than Volume
A tall 10-gallon reservoir that does not fit under your stand is worthless. Measure cabinet height with the doors open, and account for the space you need above the reservoir to remove the lid and fill it. Low-profile flat designs generally beat tall cylinders in a standard stand.
Safety Features Worth Paying For
A sealed lid. Open reservoirs collect dust, salt creep and evaporate on their own.
A fill port you can reach without pulling the reservoir out.
A level window or float indicator so you can see remaining volume at a glance.
A mechanical backup float in the sump, independent of the optical sensor. Every ATO fails eventually; the backup is what decides whether that is an inconvenience or a flood.
Reservoirs and ATO Pumps Are Separate Purchases
A reservoir is a container. You still need a pump and a sensor — an optical ATO sensor in the sump, a small dosing-style pump in the reservoir, and ideally a controller that logs run time so you can spot a leak or a stuck sensor early.
Related Collections
Pair a reservoir with an ATO pump and optical sensor from our aquarium controllers range, see the HYDROS ATO kits and water level sensors, or browse all saltwater aquarium supplies.
