The fingernail test: deposits or damage
Chalky white lines and actual scratches look alike from across the room but part company under a fingertip. Hard-water marks are mineral deposits left where droplets evaporated — typically along the waterline, under the rim and down the corners — and they sit on the surface, slightly rough, sometimes thick enough to catch a nail at their edge. A scratch is a groove cut into the glass itself: slide a fingernail gently across it and the nail drops into the line and catches. That distinction decides everything, because deposits are chemistry you can dissolve, while scratches are geometry you can only polish, fill visually, or live with. Check in good light from a low angle, since both hide when you look straight on and reappear the moment the room lights hit them.
Dissolving hard-water crust safely
Mineral crust is calcium and magnesium residue, and a mild acid dissolves it. Dampen a cloth with plain white vinegar and press it against the deposit for several minutes to soften the crust, then rub in small circles and wipe the area clean. Work on the outside of the glass freely; on inside surfaces, only do this with the tank drained or the water level dropped well below the patch, and rinse the spot thoroughly before the waterline returns, so nothing acidic lingers where fish will swim. A plastic scraper helps lift thick edges on glass, while a razor-blade scraper — glass tanks only, never acrylic — makes short work of stubborn waterline bands. As you clean, keep vinegar cloths and scrapings out of the water itself.
What can honestly be done about scratches
Real scratches demand honesty about effort and reward. Fine, shallow scratches in glass can be polished with cerium oxide paste worked with a felt pad, but it is slow, messy, and strictly a job for an empty, drained tank — the slurry must never reach aquarium water. Deeper grooves that catch the nail firmly will not polish out in any reasonable time. Acrylic is the forgiving material here: acrylic scratches buff out with dedicated acrylic polishing kits, working through grades of abrasive, and acrylic owners enjoy one mercy — many scratches nearly vanish when the tank is full, because water fills the groove and matches the material's optics. Glass shows the same kindness to a lesser degree, which is why many keepers rotate a scratched pane to the back and call it done.
Stopping the white lines from coming back
Prevention is mostly drip discipline. Wipe the outside glass and rim dry after every water change, because each drip that evaporates leaves its minerals behind in exactly the band you keep scrubbing. Keep the waterline steady where possible — a level that rises and falls over the same strip deposits a fresh line at every height — and dry your hands before reaching in, since wet fingers print the front pane. When a tank does come down for a full de-crust or a scratch polish, that is a multi-job day: seals inspected, equipment soaked, glass treated. Listing those jobs on KinsLe through Care Teams lets the whole household share the teardown instead of one person remembering every step.