How should I top up evaporated aquarium water?

Why this happens

Evaporation removes water but leaves salts, minerals, nitrate and other dissolved material behind. Replacing the lost volume with highly mineralised water repeatedly can raise GH or TDS, while topping off a marine tank with saltwater pushes salinity upward.

• Warm, dry and strongly aerated rooms lose water faster.
• Open-top tanks and cooling fans increase evaporation.
• Tap water may add minerals at every top-up.
• Automatic top-off faults can change level or salinity quickly.

Top-up water is not a water change because it does not remove accumulated waste. A tank can remain visually full while nitrate, minerals or salinity drift outside the livestock’s normal range.

What to check first

• How much volume is lost between water changes?
• What are the GH, KH and TDS of the source and aquarium?
• For marine tanks, is salinity measured with a calibrated instrument?
• Is an automatic top-off reservoir protected from siphoning or sticking?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Mark the normal water levelUse a discreet line so evaporation is separated from leaks and overfilling.
2. 2. Prepare suitable replacement waterCondition tap water or remineralise purified water according to the system plan.
3. 3. Add slowlyAvoid temperature shock, substrate disturbance and abrupt salinity change.
4. 4. Continue normal water changesUse scheduled changes to export dissolved waste even when top-offs are frequent.

How to interpret the response

Stable water level and chemistry confirm the method is working. A steady rise in conductivity, hardness or marine salinity means the replacement water or maintenance plan needs adjustment.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not add saltwater for routine marine evaporation, pour untreated chlorinated water directly onto livestock, use top-up to avoid water changes or ignore a suddenly increased loss rate that could indicate a leak.

When it is urgent

Rapid water loss, exposed heaters, salinity outside the safe range, a stuck top-off pump or water appearing outside the aquarium requires immediate action.

How to prevent it next time

Record weekly top-up volume, cover the tank where suitable, maintain auto-top-off sensors and compare source-water minerals with the livestock’s target range.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.