Guide · 4 min read · 620 words
Sizing an ATO reservoir for a reef tank
How we wrote this
We write from primary sources, in this order of preference: peer-reviewed marine science, then Randy Holmes-Farley's Reefkeeping chemistry series, then named talks and manufacturer lab data, and only then named forum contributors. Plain community consensus is used last and labelled as such.
Where the hobby genuinely disagrees, we present both sides and name the proponents rather than pretend there's one answer. Every guide is reviewed for accuracy at least quarterly, and every correction we make is logged publicly. Read the full method.
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. The evaporation rate calculator turns that measured daily loss into a reservoir size for your refill cadence.
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.
Nano ATO: AutoAqua Smart ATO Micro — $65 →
Mid-size ATO: Tunze Osmolator 3155 ATO — $190 →
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.
People also ask
- How often should I refill the ATO reservoir?
- Pick a cadence (weekly, fortnightly) and pick a reservoir big enough to cover it. Most reefkeepers refill weekly because they're already doing water changes weekly.
- Can I use the ATO reservoir to dose kalkwasser?
- Yes. Mix calcium hydroxide into the RODI reservoir at saturation (~2 tsp per gallon). The ATO drips kalk in alongside the evaporation replacement. Cheapest way to add alkalinity and calcium.
- What happens if the ATO sensor fails?
- Without a failsafe sensor, a stuck-on ATO pumps the reservoir into the tank, dropping salinity to whatever the water-to-saltwater ratio works out to. A stuck-off ATO lets salinity climb as evaporation continues. Always run a two-sensor (optical + float) failsafe configuration.
Equipment mentioned in this guide
AutoAqua ATO Micro for nano, Tunze Osmolator 3155 for mid-size. Brute can for the reservoir. Add kalkwasser to the reservoir for cheap Ca + alk dosing along with the top-off.
AutoAqua Smart ATO Micro
Best small-tank ATO. Dual-optical sensor failsafe.
View on Amazon →
Tunze Osmolator 3155 ATO
Mid-size ATO with optical + float failsafe.
View on Amazon →
Rubbermaid Commercial Brute 14 gal container
Standard ATO reservoir. Food-grade plastic.
View on Amazon →
Kalkwasser (calcium hydroxide) — Mrs. Wages or Two Little Fishies
Drip in top-off for cheap Ca + alk + pH boost.
View on Amazon →
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Sources & references
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