Which model is it?
Photograph the model label, write down the wattage and the parts that come with it, and compare them with the manual and the model page. One product name can cover several generations with different parts and supplies.
For a filter, check the body, lid, valves, baskets, intake and outlet all belong to the same model. For a light, check the power supply, mounting legs and controller are there. If you cannot tell exactly which model it is, you cannot be sure you can get parts for it.
Leak-test a used tank, then move it empty
Before you fill it, look along every seam for gaps, peeling or cut silicone, and check the glass for chips at the corners. For the test, set the tank on a flat, level surface that carries the whole base, somewhere a leak does no harm, fill it to the top and leave it for a day. Dry seams after a day show the tank holds water now; they do not promise it will for years, so keep checking the seams once it is set up.
Then empty it completely, gravel included, before you move it. Aqueon’s moving guide says never to lift or carry a tank with water or gravel in it, because the weight and sloshing water can damage the tank and its seals. Its dimensions sheet asks for a stand sized and built for the tank and its full weight.
Damage to walk away from
- Cracked housings or glass, damaged cords or plugs, or signs of water inside an electrical part.
- Home-made changes, taped or spliced wiring, or a power supply from something else.
- Nicked seal faces, missing valves, or parts you cannot be sure belong to that model.
- A heater whose water-line marks have worn off, or whose model you cannot identify.
Walk away from these: a damaged electrical part is replaced, not repaired, and a seller’s word that it worked is not enough. If you cannot tell whether a part belongs to the model, ask the maker before you use it. Electrical setup
Missing parts and what they cost
Check the hoses for cracks and stiffness, the seals and O-rings for flat or nicked spots, and which media and parts are missing. Look the replacement parts up by model in the manual: a seal, basket or controller has to be made for that filter or light, however similar another one looks.
Add the price and the missing parts in the first-year cost calculator. A used filter that needs a new head seal, impeller and media can cost close to a new one.
Was it used for sick fish?
Ask what the tank or filter was used for, and whether fish died or were treated in it. Equipment and water can carry fish diseases from one tank to another, as UF/IFAS notes. Routine cleaning does not disinfect anything. If disease is suspected, ask a fish vet how to disinfect it, and check what the maker allows on the material: some plastics, seals and silicone are damaged by strong chemicals. Routine cleaning
Setting it up again
Set it up the way its manual says: assemble it, prime the filter, let a heater stand in the water for its wait before plugging it in. Then watch it for a day for leaks, odd noise and steady running before any fish depend on it. Keep the model name and the parts you replaced; you will want them for troubleshooting later.