A second-hand tank is glass, silicone, and somebody else's history. The checks that protect your home and your fish happen at your place, before the tank is added: a fill test under weight, a close inspection of seals, glass, and stand, a plain-water rinse, and questions asked while the seller can still answer them.
Questions to Ask the Seller First
Ask before you pay, and write the answers down. Ask what the tank held last, and for how long. Ask what was used in it: any treatments, algaecides, or cleaning chemicals, since residues can linger in silicone and harm later inhabitants. Ask why it is being sold, how long it has stood empty, and whether it has ever leaked, been resealed, or been moved while full.
Ask about the stand as well: whether it was made for this tank, and whether the tank has always sat on it. None of these answers needs to end the purchase on its own, but evasive answers about leaks or chemicals are information too. Keep the answers with the tank's record, since a tank you later pass on deserves the same honesty from you.
The Outdoor Fill Test, Left for a Day
Set the tank on its stand outdoors, or in a garage or utility space where a leak can do no harm, on a level surface. Fill it completely, to the working level it will run at indoors, and leave it full for a full day. A short fill that holds for an hour proves little; seals under sustained weight and water pressure reveal themselves slowly, and a weep that starts overnight is exactly the failure you are testing for.
During the day, check the base and the floor around it at intervals, and run a dry hand or a sheet of plain paper along every seam and corner. Paper shows damp early, before a drip forms. Note the water level against a mark when you fill it, so a falling level is measured rather than suspected. Inspect the seals again while the tank is full and under load: lifting edges, bubbles inside the silicone bead, and cloudy or soft sections all count against the tank.
Glass and Stand Inspection
With the tank still full, examine the glass in good light. Look for deep scratches you can feel with a fingernail, which weaken the pane and catch algae forever after; chips on the edges and corners, where strength matters; and any crack, however fine, which ends the discussion at once. Check the braces and the rim frame for cracks or looseness, and sight along the panes for bowing beyond a slight, even curve.
Then judge the stand as structure, not furniture. It should be level, rigid under a firm push at the top corner, and free of water damage, swelling, or soft patches, especially around the feet. The tank's full footprint must sit supported, with no overhang. A doubtful stand is replaced rather than trusted, since the fill test only proves the combination you actually tested; changing the stand afterwards restarts the proof.
Rinse, Decide, and Record
Empty the tank where the water can drain safely, and rinse it thoroughly with plain water only: no detergents, no household cleaners, nothing scented. Wipe the panes with a soft cloth or a new sponge kept for the tank, rinse again, and let it dry. If the seller's answers mentioned chemicals used inside the tank, rinse a further time and let it stand filled with plain water for some hours before a final rinse and dry.
Decide from the whole picture: a full day without a weep, sound seals under load, glass free of cracks and serious chips, and a stand you would trust with the weight. Record the checks, the seller's answers, and the date of the fill test, so the tank joins your setup with a known past.
KinsLe files that record with the tank from the start, since a Pet Passport keeps the fill-test result and the seller's answers together, ready when the tank is stocked or passed on.