A second-hand tank is one of fishkeeping's great bargains, but it arrives carrying an unknown history: mineral crust, algae baked onto glass, possible disease organisms, and whatever the previous owner last cleaned it with. Cleaning it safely is therefore less about making it sparkle and more about removing residues and risks in the right order, then proving the empty box holds water before it ever meets a fish.
Start outside, and strip it back to bare glass
Work in a garden, garage, or bath where spills do not matter, and remove everything loose first: old gravel, ornaments, filter parts, and any perished background. Much of the grime on a used tank is simply dried algae and limescale, which plain warm water, a soft cloth, and an algae scraper made for the tank's material will shift without chemicals. Never use metal blades or scouring pads on acrylic, and avoid them on glass seams too. Stand the tank on a soft, level surface while you work so grit underneath cannot crack the base as you lean into a stubborn patch.
Use vinegar for mineral deposits, and no soap ever
White vinegar is the safe workhorse for the white crust around the waterline and lid. Soak paper towels in it, lay them against the deposits for a while, and the mineral softens enough to wipe away; a paste left on heavier build-up handles the corners. The hard rule is what you must not use: washing-up liquid, glass cleaners, and household sprays all leave surfactant and chemical residues that cling to glass and silicone and can kill fish long after the tank looks rinsed. If the tank's history is genuinely unknown and you feel disinfection is needed, research a fish-safe method carefully and rinse exhaustively afterwards, because the rinsing, not the product, is what makes it safe.
Rinse until there is nothing left to rinse
Rinse every surface repeatedly with clean water, changing the water rather than swishing the same bucket around, until there is no vinegar smell and no slick feeling under your fingers. Drain and refill at least a few times for a tank that had heavy deposits, and let the final rinse sit briefly before a last emptying. Dry the glass with a clean cloth and inspect it in good light as you go, because water hides the fine scratches and chips that a dry pane reveals. A chipped corner or a deep scratch on a load-bearing pane is a reason to walk away from the tank, not clean it harder.
Inspect seams and leak test before it comes indoors
Run a fingertip along every silicone seam feeling for lifting, bubbles, or sections gone hard and cracked, since tired silicone is the usual reason a used tank was sold. Then fill the tank completely, outdoors or in the bath, mark the water level, and leave it for at least a full day, checking the seams and the surface beneath for any dampness. Only a tank that passes dry outside earns a place on a stand indoors. Wipe the outside glass last with a cloth dampened with plain water, and position the empty, proven tank before filling it for cycling.
Noting the vinegar clean, seam check, and successful leak test in KinsLe Pet Passport gives this recycled tank a documented fresh start alongside its future inhabitants' records.