Seams first: where used tanks actually fail
Glass rarely fails; silicone does. Go over every internal seam slowly, in good light, pressing gently with a fingertip. Healthy silicone is a smooth, continuous bead firmly bonded to both panes, while failing seams show lifting edges, trapped bubbles, cloudy patches or places where the bead has thinned to a film. Check the bottom seams twice — they carry the full weight of water and substrate pressing down and outward, and they are hidden forever once the tank is set up. A tank that has been resealed is not automatically a bad buy, but fresh silicone smeared over old, or beads of uneven width, tell you the repair was cosmetic — and cosmetic is not what holds back a wall of water above your floor.
Glass, corners and bracing
Chips matter according to where they live. A shallow shell chip on an outer corner edge may be purely cosmetic, while any chip, crack or star fracture on a corner seam, near the bottom, or radiating from an impact point is a walk-away, because that is where the structure works hardest. Run your eyes along each pane for scratches too: fine ones vanish underwater, but deep scratches catch the light forever and cannot be polished out of a filled tank. If the tank has a plastic frame or centre brace, check the brace for cracks and sun-brittleness; on larger tanks that brace is structural, and a failed one lets the long panes bow outward under pressure. Rimless tanks demand extra care at their edges, which are both the structure and the most chipped part of most used examples.
The fill test is not negotiable
Never trust a dry inspection alone. Fill the tank completely, outside or somewhere a leak is an inconvenience rather than a disaster, on a surface that is flat and level — a twisted stand or uneven floor stresses seams in ways that can mimic failure and, given time, cause the real thing. Leave it full for a full day at least, then run a dry paper towel slowly along every seam and corner joint; the towel finds slow weeps that eyes and hands walk straight past. Watch the waterline against the glass for level while it stands, and check that no long pane bows alarmingly once the tank is under full pressure.
History, cleaning and the equipment bundle
Ask the seller what lived in it. A tank that held reptiles may have heat-damaged trim and dried, stressed silicone; one treated with copper-based medication can carry residues that harm shrimp and other invertebrates long after; and any tank a previous owner scrubbed with household detergents is suspect, because soap traces are poisonous to fish. Clean a second-hand tank yourself with plain water and, for mineral crust, white vinegar and a non-scratch pad — never detergents. Treat bundled equipment with the same suspicion: replace an old heater outright, since a heater stuck on is the classic used-kit disaster, and run filters wet before fish depend on them. If a seam verdict is beyond your eye, a trusted local aquarium shop found through the KinsLe Pet Services Directory can inspect and reseal a tank properly — far easier to arrange before the tank is full of water and fish than after.