How does evaporation affect salinity in a saltwater tank?

Evaporation removes pure water and leaves salt behind, so the tank's salinity slowly climbs unless you top off with RO/DI freshwater. A 75-gal reef losing 1 gal/day raises salinity by ~0.4 ppt per day if you forget to top off — enough to stress corals after 48 hours. ATO automation removes this risk.

Why does winter evaporate more than summer?

Indoor humidity. Modern homes run 20–35 % RH in winter due to forced-air heating, and 50–70 % in summer. Lower humidity = higher evaporation rate at the same temperature. The same tank can evaporate twice as much in January as in July.

Should I run a fan to cool the tank?

Yes, in summer, if your tank runs warmer than 80 °F. A small clip-on or USB fan blowing across the sump surface drops tank temperature 3–4 °F via evaporative cooling. You'll roughly double your ATO refill — plan reservoir accordingly. The trade-off is usually worth it vs running a chiller.

Does evaporation rate change as the tank seasons in?

Yes — the first 4–8 weeks of a new tank often evaporate slightly more because the rock and substrate are still releasing trapped air. After the tank stabilises, evaporation tracks predictably with the formula above.

How big should my ATO reservoir be?

At least 5–7 days of evaporation. That gives you headroom for vacations, slow weeks, or anything that breaks your daily top-off routine. The calculator's 'recommended ATO' figure is 7 days × daily evaporation.

Calculator

Aquarium Evaporation Rate Calculator

Name: Aquarium Evaporation Rate Calculator
Author: reefcalcs

Evaporation isn't just a top-off chore — it's the largest variable in your tank's salinity stability and the single biggest driver of how often you need to refill the ATO. This calculator gives you a realistic daily and weekly evaporation estimate based on surface area, water-to-room temperature delta, room humidity, top cover, and air movement at the surface.

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Evaporation rate

Surface length (in)

Surface width (in)

Tank temperature (°F)

Room temperature (°F)

Relative humidity (0–1)

0.50 = 50 %. Dry winter homes drop below 0.30.

Top cover

Surface air movement

Evaporation per day

1.04gal

Per day (litres)

3.94L

Per week

7.28gal

Surface area

6.00ft²

Recommended ATO reservoir (7-day)

7.3gal

Buffer to top-off without daily refills.

Calibrated against the BRS observation that a 40-gal open-top reef in a 72 °F / 50 % RH room evaporates ~0.5 gal/day. ASHRAE-style pool-evaporation formula simplified for hobby surface areas.

How this is calculated

Surface area (ft²) = length_in × width_in / 144

gal/day = surface_ft² × ((Tw − Ta) × 0.024 + (1 − RH) × 0.06)
          × cover_factor × air_factor

Cover factors:
  open top       1.00
  mesh / screen  0.85
  glass canopy   0.30

Air movement:
  none           0.85
  normal         1.00
  surface fan    1.40

Calibration: 24″×16″ open top, Tw 78 °F, Ta 72 °F, RH 50 %, normal flow
            → 0.46 gal/day (matches BRS observation of ~0.5 gal/day on
              a 40-gal open-top reef in similar conditions).

The formula is a simplified ASHRAE pool-evaporation model with hobbyist coefficients fit to published observations. It's accurate enough to size an ATO reservoir confidently; it's not laboratory-grade.

Three variables matter more than the formula suggests: a covered top cuts evaporation by ~70 %, a surface fan increases it by ~40 % (and drops tank temp 3–4 °F via evaporative cooling — useful in summer), and winter dry air can double summer evaporation in the same tank because room RH drops from 60 % to 25 %.

FAQ

How does evaporation affect salinity in a saltwater tank?: Evaporation removes pure water and leaves salt behind, so the tank's salinity slowly climbs unless you top off with RO/DI freshwater. A 75-gal reef losing 1 gal/day raises salinity by ~0.4 ppt per day if you forget to top off — enough to stress corals after 48 hours. ATO automation removes this risk.
Why does winter evaporate more than summer?: Indoor humidity. Modern homes run 20–35 % RH in winter due to forced-air heating, and 50–70 % in summer. Lower humidity = higher evaporation rate at the same temperature. The same tank can evaporate twice as much in January as in July.
Should I run a fan to cool the tank?: Yes, in summer, if your tank runs warmer than 80 °F. A small clip-on or USB fan blowing across the sump surface drops tank temperature 3–4 °F via evaporative cooling. You'll roughly double your ATO refill — plan reservoir accordingly. The trade-off is usually worth it vs running a chiller.
Does evaporation rate change as the tank seasons in?: Yes — the first 4–8 weeks of a new tank often evaporate slightly more because the rock and substrate are still releasing trapped air. After the tank stabilises, evaporation tracks predictably with the formula above.
How big should my ATO reservoir be?: At least 5–7 days of evaporation. That gives you headroom for vacations, slow weeks, or anything that breaks your daily top-off routine. The calculator's 'recommended ATO' figure is 7 days × daily evaporation.

Share / embed this calculator

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Sources & references

01
Bulk Reef Supply — Get saltwater tank evaporation right (Ep 20)
https://www.bulkreefsupply.com/content/post/5-minute-saltwater-aquarium-guide-ep20-evaporation
02
ASHRAE Handbook — Indoor pool evaporation methodology (Carrier model)
https://www.ashrae.org/technical-resources/ashrae-handbook
03
Reef2Reef — Evaporation rate megathread (hobbyist data)
https://www.reef2reef.com/threads/how-complex-is-it-to-calculate-the-evaporation-rate-of-a-typical-medium-size-reef-tank.1052261/

Last reviewed 2026-05-25