Hands first: edges, seams and surfaces
Pick the piece up and feel it properly, because eyes miss what fingertips catch. Run your fingers along every rim, fin and moulded detail looking for sharp points, brittle fins of plastic and rough seams where two moulded halves meet — all of these can slice a long fin or scrape a flank during a startled dash. Flex thin plastic gently; a piece that creaks, whitens at the bend or feels soft and tacky is degrading and should be refused outright. Check painted finishes the same way: rub hard with a damp thumb and look for colour transfer, bubbling or flakes. Paint that is letting go in your hand will let go in the water, sheet by sheet, and the fish will eat the flakes.
Hollow pieces and hidden traps
Castles, caves and novelty ornaments hide their worst faults inside. Shine a light into every chamber and doorway and judge each opening against the fish you actually keep: a hole a curious cory can enter but not turn in is a trap, and a hollow piece with a single small opening can hold a pocket of stale, oxygen-poor water that pours back into the tank when the piece is moved. Tap hollow ornaments and listen for cracks, and check that pieces sit flat and stable rather than rocking, because a toppling ornament can pin a bottom dweller. Solid, simple shapes from a previous keeper are almost always safer than elaborate hollow ones, however charming the castle looks in the listing photograph.
Ask about the piece's past life
Decor remembers its previous tank. Ask the seller what lived alongside it and whether that tank was ever treated for disease, because porous materials — lava rock, bogwood, terracotta, unsealed stone — absorb substances from the water and can release traces long afterwards, and some fish medicines are exactly the chemicals sensitive invertebrates cannot tolerate. Rocks deserve their own question: an unknown stone may be loaded with carbonate that pushes pH upward, or worse, carry metal ore flecks. A drop of ordinary household vinegar on a dry rock that fizzes vigorously is telling you the stone will dissolve and harden your water — fine for some African cichlid setups, wrong for a soft-water community. If the seller cannot say where a rock came from, treat that as your answer and leave it.
Clean, soak and quarantine before the main tank
Anything that passes inspection still earns a probation period. Scrub it in plain water with a stiff brush, soak it for several days with the water changed daily, and watch the soak water for colour, film or smell — all are grounds for rejection even at this late stage. Wood that will not stop leaching or turning its soak water to tea may still be usable, but you want to know before it tints the display tank. Rest the cleaned piece in a quarantine tank or a spare tub with a test plant or a few hardy snails before it meets valuable stock. Put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.