Measure a loss over a known interval
Daily aquarium evaporation rate is observed evaporative water loss divided by elapsed days. It is not a fixed percentage assigned to every fish tank. Start with an identifiable operating level, observe a representative interval and measure the water needed to restore that same level, while accounting for any other additions or removals. The quality of that water-balance record matters more than the calculator's decimal places.
This guide focuses on obtaining the rate input. The separate ATO-reservoir guide explains how a chosen planning interval and allowance affect reserve quantity. Neither method diagnoses a leak or guarantees unattended operation. An unexplained sudden loss needs investigation rather than being accepted as a new normal evaporation rate and automatically compensated with a larger water supply.
Choose a repeatable operating-level reference
Use a reference that represents the same normal operating state at the start and end. In a connected system, different chambers can respond differently to return-pump operation and drain-back. Changing which equipment is running can change the visible level without changing total water volume. Follow equipment instructions and avoid unsafe pump or heater operation merely to obtain a convenient reading.
Record the reference location and relevant operating conditions, such as the usual cover arrangement and room conditions. Do not compare an after-maintenance level with a different pre-maintenance mark. A marked reservoir or measured refill container can be more repeatable than judging a curved waterline by eye, but whichever method is used must refer to the same state.
For manual replacement, measure cumulative top-up water
Suppose a system is restored to the same reference level with measured freshwater additions of 0.8 L, 1.1 L and 0.9 L over an exact two-day interval. With no other water movements and equal start/end levels, the observed loss is 2.8 L and the average is 1.4 L/day. The number of top-up events is not the denominator; elapsed time is.
If the final level remains lower, the measured additions alone understate the total loss. If the final level is higher, they overstate the loss relative to the reference. Restore and measure consistently or record a defensible start/end volume correction. Do not fill above the intended operating level simply to make a collection period end at a round number.
For an ATO, use a reservoir inventory rather than display-tank drop
An automatic top-off system may keep the display or sump level nearly constant while its freshwater reservoir decreases. A steady controlled level therefore does not imply zero evaporation. Record starting reservoir water, measured additions to that reservoir and ending water. Assuming the reservoir only supplies the ATO and has no leakage, delivered water equals start plus additions minus end.
For example, start with 12 L, add 5 L during the interval and finish with 10 L after five days. Delivery is 12 + 5 − 10 = 7 L, or 1.4 L/day. Looking only at the two-liter decrease would incorrectly suggest 0.4 L/day. Account separately for freshwater poured directly into the aquarium, since it never passed through the reservoir.
Subtract other removals and record additions separately
Water drained for a change, removed in a sample, lost through a leak or exported in wet skimmate is not evaporation. If top-up water replaces those losses, attributing all delivery to evaporation overstates the rate. In marine systems, exported saltwater and evaporation also differ in salt balance; a top-off observation is not a salinity-management prescription.
As a hypothetical water balance, 8 L of reservoir delivery over five days includes 1 L of separately measured non-evaporative removal. With equal system start/end levels and no other movements, the inferred evaporation is 7 L, or 1.4 L/day. If the extra movement cannot be measured reliably, mark the interval as unsuitable rather than inventing a correction.
A level-drop estimate needs the correct surface geometry
For a straight-sided rectangular compartment, a level fall can be approximated as length × width × height drop in cubic centimeters, divided by 1,000 for liters. A 60 × 30 cm water surface falling 0.5 cm corresponds to 0.9 L. If that change took 18 hours, the implied daily average is 0.9 ÷ (18/24) = 1.2 L/day.
This shortcut requires an appropriate geometry and stable operating configuration. Sloping walls, multiple changing compartments, measurement error and equipment-state changes can invalidate it. It does not work by multiplying the aquarium's nominal gallons by an arbitrary daily percentage. Prefer directly measured replacement volumes when a small level difference cannot be read accurately.
Combine unequal intervals by total loss and total time
Suppose one observation records 2 L over one day and another records 3 L over three days under comparable conditions. The combined average is (2 + 3) / (1 + 3) = 1.25 L/day. Averaging the separate rates of 2 and 1 L/day would give 1.5 L/day and incorrectly give the short interval the same weight as the long interval.
Keep records from substantially different conditions separate when planning. A cover change, stronger ventilation or a seasonal room change may make an older average less representative. Several observations help describe variability, but the calculator does not perform weather prediction or automatically select a safe margin. Enter a representative observed quantity and its actual corresponding duration.
Enter a measured rate without overstating its certainty
The Aquarium Evaporation Calculator divides observed liters by observed days, then scales the rate to the planning period and any user-chosen extra allowance. For 7 L over five days, enter those matched observations rather than entering 7 L and one day. An allowance changes the planning quantity, not the original measured evaporation rate.
A zero observation can reflect a short interval, a measurement limit or an unrecorded top-up; it is not proof of unlimited reservoir coverage. Keep equipment safeguards and monitoring separate from the estimate, and verify a changed setup before relying on an old log. The useful result is a transparent measured average with stated conditions, not an assurance that future water loss will be identical.