Aquarium Evaporation Calculator

In 2026, a reef keeper's 180-gallon SPS tank runs an Innovative Marine Hydrofill ATO connected to a 10-gallon RO/DI reservoir; the Apex controller logs daily evap at 0.8 gallons, refills automatically, and alerts on the iPhone if reservoir falls below 25%. Forty years ago, the same tank was topped off manually with a measuring cup whenever the owner remembered, salinity bounced between 1.022 and 1.029 SG, and corals slowly bleached out.

The pre-automation era (1960s-1990s): Hobbyists topped off when they noticed the water line had dropped — often a week or two between refills. For reef tanks, this meant salinity creep was the dominant failure mode. Sprung and Delbeek's "The Reef Aquarium" volumes (1994-2004) established the "top with fresh, change with salt" rule but offered no automation guidance.

The first automation wave (1995-2010): JBJ's aluminum-housed float-switch ATOs ($60) entered the market in the late 1990s. Tunze Osmolator (Germany) brought reliability and precision ($150). Bob Goemans' "The Living Aquarium" (2003) and Anthony Calfo's "Reef Invertebrates" (2003) standardized 5-gallon RO/DI reservoirs as the hobbyist ATO scale. Marine hobbyists understood that salinity stability was the difference between thriving SPS and dead frags.

The smart-controller era (2010-present): Neptune Apex (Cliff Christmas, 2010) and Hydros controllers (formerly Coralvue) added optical sensors, multi-stage safeties, and cloud logging. Innovative Marine Hydrofill, AutoAqua Smart ATO, JBJ TopOff — all standardized on 12V optical sensors and silicone tubing. Freshwater planted-tank hobbyists adopted the same hardware for high-tech setups where TDS stability matters.

The physics: evaporation rate scales with surface area, vapor-pressure deficit (room humidity below tank-surface saturation), and gas-exchange-driven air flow. Henry's Law (William Henry, 1803) and the Antoine equation (Louis Antoine, 1888) underpin the chemistry. Pure H2O leaves as vapor; everything dissolved (salt, calcium, alkalinity, nitrate, TDS) stays in shrinking volume. Reef hobbyists discovered this empirically through 1990s SG drift; planted-tank keepers discovered the same with TDS climb in EI-dosed tanks.

Modern best practice: install an ATO (Tunze Osmolator, AutoAqua, Innovative Marine, JBJ) on any reef or open-top freshwater tank. Use pure RO/DI fresh water for top-off — never source water (tap), never salt water. Check refractometer SG weekly for reef tanks. Replace RO/DI cartridges per manufacturer schedule (6-12 months). Refill the reservoir before it's empty — running an ATO pump dry damages the impeller.

For the rest of your aquarium maintenance toolkit, see the Water Change Calculator, CO2 Calculator, Heater Size Calculator, and Lighting Calculator.

Sources: Julian Sprung & J. Charles Delbeek, "The Reef Aquarium" vols 1-3 (Ricordea, 1994-2004). Bob Goemans, "The Living Aquarium" (TFH, 2003). Anthony Calfo, "Reef Invertebrates" (Reading Trees, 2003). Tunze Osmolator hardware documentation. Innovative Marine ATO product specs. William Henry, "Henry's Law" (Phil. Trans. R. Soc., 1803). Louis Antoine, "Antoine Equation" (1888).