Understand what evaporates
When water evaporates, dissolved minerals remain in the aquarium. Replacing evaporation with mineralized water can slowly raise concentration, even if the tank appears to be at its original level.
Use a marked water line or consistent top-off practice so the change is gradual and easy to track.
Separate top-offs from water changes
Top-off water restores volume but does not remove nitrate or other accumulated substances. A planned water change still serves a different maintenance purpose.
Use purified water appropriate to your setup for evaporation replacement, then restore minerals through water changes or a tested remineralization routine as needed.
For your own tank
Measure top-off water separately and avoid adding minerals just to restore the visible water line. Track the level and conductivity trend over time, remembering that TDS does not identify the dissolved substances. Continue planned water changes for waste removal.
Use these as general hobby references. Test your aquarium and confirm product guidance before making changes.