Sizing an ATO reservoir for a reef tank

How to measure your evaporation rate

Mark the water line on your sump (or tank) with a paint marker. Wait 24 hours. Refill exactly to the line, measuring how much you added. That's your daily evaporation rate. Do it twice to confirm — room humidity and tank lid configuration both swing it 50%+.

Open-top reef tanks in dry winter rooms can lose 2–3 gallons per day on a 100-gal system. Mesh-lidded tanks in humid rooms lose half a gallon. Don't skip the measurement step.

Common reservoir sizes & how long they last

• 1 gallon container — 1–2 days. Useful only as supplemental.
• 5 gallon bucket — 5–10 days for a 100-gal system. Most common starter.
• 10 gallon plastic container — 10–20 days.
• 14 gallon Brute / 20 gallon Brute — 14–28 days. The vacation-proof choice.
• 32 gallon Brute — month-plus capacity. Overkill for most home setups; common in commercial.

What to buy

For the ATO controller: AutoAqua Smart ATO Micro for nano tanks, Tunze Osmolator 3155 for mid-size, or run it through an Apex controller with optical sensors if you already own the Apex.

For the reservoir: 5-gallon BPA-free buckets work fine. Brute trash cans (Rubbermaid Commercial) are the heavy-duty standard — rigid plastic, made to hold liquid, sized in 10/14/20/32 gallon. Add a fitted lid to keep dust out.

Failsafes that actually matter

• Two sensors, two technologies. Optical (light beam) + mechanical (float). If both agree the tank is full, ATO stops. Single-sensor ATOs flood living rooms.
• Reservoir on a leak-tolerant surface. Tile or vinyl floor, not hardwood. Inside a 5-gallon plastic tub if possible.
• Maximum runtime cutoff. ATO firmware should kill the pump if it runs more than X seconds continuously. Tunze and AutoAqua both have this; cheaper ATOs sometimes don't.
• Float valve in the sump. Mechanical backup independent of the ATO. If ATO pump won't stop, float valve cuts the water-line gravity feed.

Water change calculator for the related math of partial water exchanges.