Evaporated water and removed saltwater are different losses
For ordinary marine evaporation, the water leaves while dissolved salts largely remain. Replacing that evaporative loss with appropriately prepared freshwater restores water without deliberately adding another portion of marine salt. Adding newly mixed saltwater for every evaporated liter would add salt that the evaporation did not remove. This is the reason for the freshwater top-off distinction, not a generic instruction to dilute any high salinity reading.
Actual saltwater removal is different. A water change, spill or other exported liquid can remove salts along with water. The correct replacement procedure depends on what was lost, its composition and the aquarium's measured condition. This educational article explains the bookkeeping boundary. It does not prescribe a salinity correction rate, a universal replacement-water specification or an emergency response for livestock.
Use a conservative-tracer illustration
Imagine an idealized 100 L container holding a dissolved tracer at 35 units per liter, for 3,500 total units. If 5 L of pure water evaporates and none of that tracer leaves, 3,500 units remain in 95 L. The resulting concentration is about 36.84 units per liter. Returning 5 L of tracer-free water restores the original 100 L and the original 35-unit concentration.
If 5 L containing 35 units per liter is added instead, another 175 units enter. The container then holds 3,675 units in 100 L, or 36.75 units per liter. This simplified calculation illustrates why adding dissolved material for an evaporative loss can accumulate it. It is not a seawater salinity conversion: practical salinity, density, mass and volume conventions require more care than the invented tracer units.
Do not classify every falling level as evaporation
A leak or saltwater spill is not equivalent to water vapor leaving the system. Likewise, a skimmer export or a removed test sample can carry dissolved material out. The volume shown by a top-off device does not identify the cause. If the normal replacement demand changes unexpectedly, investigate the system and record intentional removals before treating the whole amount as an evaporation measurement.
A useful daily ledger separates evaporative replacement, prepared saltwater added and saltwater deliberately removed. It can also note unusual splashing or maintenance. These categories help explain why water level and salinity observations may diverge. They are not a reason to ignore a suspected leak or allow an automatic device to compensate indefinitely for unexplained water loss. Seek appropriate support when the cause cannot be established safely.
Keep water quality and water quantity as separate checks
Calling a supply freshwater only states that it is not deliberately mixed marine saltwater. It does not certify purity, treatment performance or compatibility. Verify the intended preparation according to the equipment and aquarium requirements. A source marked RO/DI still needs the relevant operating and quality checks; a production-time calculation cannot establish what dissolved substances remain in the output.
The top-off reservoir's purpose should also be explicit. A reservoir containing additives is no longer the plain-water case described by a simple evaporation illustration. Its concentration and delivery implications require separate product guidance. Do not copy a plain-water replacement calculation into an additive-dosing workflow or assume that a larger reservoir makes that workflow safer. The contents and function of the reservoir matter as much as its capacity.
Measure level and salinity independently
A level sensor observes its configured waterline condition, not salinity. A salinity instrument observes a sample using its own method and calibration convention, not how much water evaporated yesterday. Keep these observations independent so one can help reveal problems in the other. A stable level does not prove a stable salt balance, especially when some water has been exported and replaced by a different source.
Use the salinity instrument's instructions, including its calibration and temperature requirements. Record the displayed scale instead of copying a number without units. Specific gravity and practical salinity are not interchangeable labels. If two readings disagree, investigate the sample, method and device rather than changing the water until one preferred display appears. This article intentionally provides no conversion from a reading discrepancy to a correction dose.
Plan the evaporative supply from observed loss
Suppose a separate, representative record shows 8 L of evaporative replacement over four days. The observed average is 2 L per day. A five-day planning period therefore projects 10 L before allowance. With an independently selected 25 percent planning allowance, the calculator returns 12.5 L. Neither number includes water changes, saltwater exports or any chemical addition, which need their own ledgers.
If a reservoir has 15 L of usable water, dividing by the allowance-adjusted 2.5 L per day gives six projected days. This is a steady-rate arithmetic estimate, not an unattended-operation guarantee. A sensor failure, pump problem, siphon, leak or changing room condition can invalidate it. Follow the actual top-off equipment's installation and safety instructions rather than interpreting the supply estimate as a complete system design.
Avoid turning a top-off rule into a correction recipe
The statement that evaporative loss is normally replaced with suitable freshwater does not mean that every marine aquarium with a low level should immediately receive a calculated freshwater addition. First identify the loss and measure the relevant conditions. If saltwater was removed, simply restoring level with freshwater changes a different balance. The original evaporation assumption no longer describes the whole event.
Do not add dry salt directly to an occupied aquarium to compensate for an uncertain ledger or an unexpected test result. A salt-mix calculator scales a separate preparation recipe; it does not choose a safe corrective treatment. If the aquarium's condition is concerning, use qualified guidance and your actual measurements. A volume-only estimate cannot determine an appropriate correction speed or assess the response of animals.
Maintain a record that explains the replacement
Write the intended reason next to each addition: evaporative replacement, exchange refill or another documented procedure. Include the source identity and the measurement basis. Over time, this prevents a routine top-off from being confused with a water change and makes unusual demand easier to notice. The record need not be elaborate, but it must preserve the distinction between water-only loss and saltwater export.
The Aquarium Evaporation Calculator helps project one of these categories from observed loss and elapsed days. It does not infer salinity, identify leaks or choose reservoir chemistry. Use it alongside independent level and salinity checks, not in place of them. The practical answer is appropriate freshwater for verified ordinary evaporation, with every other kind of loss considered separately rather than hidden inside a single replacement number.