The short answer
That chalky white line above the waterline is limescale — dried mineral deposits left behind as water evaporates. Remove it by wiping the empty, exposed glass with a cloth and a little vinegar, then rinsing thoroughly before the water goes back up. The acid dissolves the minerals; the rinse makes sure none reaches your fish.
Why it forms
As water evaporates it leaves its dissolved minerals behind on the glass, and over time those build into a hard white crust at the old waterline. Hard water areas get it fastest. It’s purely cosmetic — a sign of evaporation, not a water-quality problem — but it’s stubborn because plain water won’t shift it.
The safe method
- Lower the water first. Do this during a water change, or on an empty tank, so the scaled band of glass is above the waterline and dry.
- Apply a little vinegar. Dampen a cloth or paper towel with white vinegar and wipe the deposits, or hold the cloth against heavier spots for a minute to soften them. A plastic blade scraper helps lift the worst.
- Rinse thoroughly. Wipe the area down with clean water several times so no vinegar residue remains, then refill.
Preventing it coming back
Limescale is really an evaporation problem, so slowing evaporation slows the scale — a lid cuts it right down. Topping up before the line dries and crusts also helps. See how to reduce evaporation, and for cleaning ornaments the same scale settles on, see cleaning aquarium decorations. Browse maintenance gear for scrapers and pads, and follow how to do a water change to fit this into your routine.