Buying a second-hand aquarium: the steps to take

Is a used fish tank actually safe?

Most of the time, yes. A glass aquarium is glass panes held together by silicone, and neither has a sell-by date. What fails is usually down to how the tank was treated: moved while full, stood on an uneven floor, scraped along the seams, or left dry in a hot loft for years.

A listing photo tells you none of this, and most advice stops at "check for leaks". This guide goes through the whole process in order, including the bits people find out the hard way: the cabinet, the floor and the plug socket.

Step 1: Questions to ask before you go

A few messages can save you a wasted trip. Ask:

• How old is it? A rough answer is fine. "No idea, it was in the garage" tells you something too.
• Why are you selling? Upgrading or moving is normal. Vague answers deserve a follow-up.
• Has it ever leaked or been resealed? A professional reseal isn't a red flag. A DIY patch over one corner might be.
• Is it running now, and what was kept in it? A tank still holding water is the best evidence you can get.
• Any disease or unexplained deaths recently? This affects what you can safely reuse.
• Was copper-based medication ever used? Copper is widely reported to linger in silicone, substrate and decor. It matters most if you plan to keep shrimp or snails.
• Was it ever moved with water or gravel inside? That twists the frame and strains the seams.
• Is the cabinet included, and is it the matching one? Many tanks are designed for a specific stand.
• Glass or acrylic, and what equipment comes with it? Filter, heater, light, lid, and roughly how old each is.

Then ask for photos or a short video of it full and running, with close-ups of the corners and inside the cabinet.

Step 2: Inspect it in person

Take a torch, a spirit level (a phone app will do) and some kitchen paper. If the tank is still full, inspect it full: it's your only chance to see it holding water.

Silicone seams

Each seam has two parts: the thin layer of silicone between the panes, which does the structural work, and the visible bead along the inside corner. Both should be continuous and firmly stuck to the glass.

• Usually cosmetic: discolouration or black and green staining. Algae and minerals stain silicone without weakening it.
• Not cosmetic: silicone lifting away from the glass, visible gaps, bubbles or channels running towards the outside edge, or nicks from a scraper blade.

Press the inside bead gently with a fingernail. Good silicone feels rubbery and stays put; brittle silicone, or a bead that lifts like old tape, is a warning.

Glass, edges and acrylic

Fine scratches on glass are mainly cosmetic, but treat them as permanent. Chips on edges and corners are more serious, because edges are where glass is most likely to crack under load. A chip on the bottom edge of a large tank is a reason to walk away.

Acrylic scratches easily but can usually be polished. What you don't want is crazing: a network of fine cracks, often near joints or holes, which tends to spread.

Bracing

Many glass tanks have glass strips around the top rim and, on longer tanks, a centre brace. Check for cracks and make sure nothing has been removed. A missing centre brace on a tank designed with one is a genuine risk, because the long panes are no longer held against the water pressure. If they bow outward when full, don't buy it.

The cabinet or stand

The cabinet gets overlooked, and it often fails before the tank. Swollen, crumbling or soft chipboard means water damage, and swollen board loses strength. Check it is square, the doors line up and the top is level. A stand that isn't level puts stress into the glass.

Equipment

Ask for the filter to be switched on and listen for rattling or grinding, usually a worn impeller. Check the heater glass for cracks or water inside, the light works, and the lid isn't warped. Note model names so you can check spare parts are still sold.

Step 3: Transport it empty

Never move a tank with water or substrate in it, not even "just a bit at the bottom". The weight flexes the frame and can break seals you won't notice until it's full again. Empty it completely and carry it by the base, not the top rim or bracing.

In the car, keep it flat with the base supported along its whole length, pad the corners with blankets, and put nothing on or in it. The moving house with an aquarium guide covers the same handling principles for an established tank.

Step 4: Leak test before it goes indoors

Do this before the tank ever sits in its final spot. Choose somewhere a leak won't matter: a patio, a garage floor or a garden path. It must be level and flat, because an uneven surface can stress the glass and create the very leak you're testing for. A sheet of polystyrene or a foam mat under the base helps on rough ground.

1. Fill it to the top with plain tap water.
2. Mark the water line with a strip of tape.
3. Leave it for 24 hours at the very least, ideally 48 to 72.
4. Run kitchen paper along every seam and the bottom edges. Paper shows a slow weep that your eyes will miss.
5. Check whether the level has dropped beyond what evaporation would explain.

A dry tank after three days is a good sign, not a lifetime guarantee, so keep an eye on the floor around the cabinet for the first few weeks.

Step 5: Clean it without soap

Don't use washing-up liquid or household cleaners. Residues are hard to rinse off and toxic to fish. Plain water and a sponge that has never touched cleaning products deal with most dirt.

For limescale lines, use white vinegar: wipe it on, leave it to soak, then scrub. Keep metal blades away from the silicone, use acrylic-safe pads on acrylic, and rinse thoroughly.

What about bleach?

The honest trade-off: very dilute bleach kills many pathogens, and some keepers use it when a tank's history is unknown or the seller mentions persistent deaths. But it must be rinsed out completely, strong or long contact isn't kind to silicone, and it doesn't guarantee against every hardy bacterium.

If you use it, use plain unscented bleach, well diluted, for a short contact time. Rinse repeatedly, add dechlorinator to the final rinse, and let the tank air-dry fully. For a tank that held healthy fish, water, vinegar and thorough drying is usually enough: most fish parasites need a host and don't last long in a dry tank.

Step 6: Decide what to keep and what to replace

The tank and cabinet are what you're really paying for. Judge each extra on its merits:

• Heater: the item most worth replacing if its age is unknown, because a heater that sticks on can cook a tank. If you keep it, test it in a bucket of water beside a separate thermometer for a few hours while you watch.
• Lighting: old fluorescent tubes dim long before they fail, so plants may struggle. Check LED units light evenly.
• Filter: usually worth keeping if it runs quietly; impellers for popular models are cheap. Replace sponges and pads if their history is unknown.
• Airline and tubing: old tubing goes stiff and cracks. Replace it.
• Used substrate: plain gravel or sand can be washed and reused if you trust the source. Used aquasoils have usually broken down and are rarely worth having. If copper was used, start fresh. See the substrate guide.
• Decor and rock: hard, non-porous items clean up well. Porous rock and wood can hold whatever was in the old tank.

Step 7: The used filter media question

If the tank is still running when you collect it, the seller's filter media holds the bacteria that make a tank safe for fish. Kept wet in tank water and back in a running filter within a few hours, it can seed your set-up and shorten the cycle, the same principle as the established filter method.

The catch is that media can carry disease as well as bacteria. Only use it if you trust the source and the tank was genuinely healthy: no recent deaths, no ongoing treatment. If in doubt, start fresh and run a proper cycle. Either way, test rather than assume. The nitrogen cycle guide explains what you're waiting for.

Resealing: when it's worth it, and when to walk away

A tank with tired but intact silicone can be resealed: cut out the old inner bead, clean the glass, lay a fresh bead of aquarium-safe silicone (100% silicone, no mould inhibitors) and let it cure as long as the tube says. On a cheap small or medium tank, that's often worth a weekend.

It's different when the structural layer between the panes has failed, a pane is cracked, or the tank is large. Rebuilding means taking the tank apart and re-bonding the glass, a skilled job. For a big tank with a failing seam, walk away unless it's nearly free and you know what you're doing.

Bargain or money pit?

Prices vary too much by size, brand and area for useful figures, but the pattern is consistent. A good buy is a sound tank and matching cabinet, ideally still running, from someone upgrading or moving. Working equipment is a bonus.

A money pit needs resealing, sits on a swollen cabinet, and comes with gear you'll replace anyway. Add up what it would cost to make usable and compare that with a new kit of the same size. The fish tank setup cost guide gives you new prices to compare against.

Safety: floor weight and electrics

How heavy it will be

Freshwater weighs about 1 kg per litre, so a 200-litre tank holds roughly 200 kg of water alone. Add the glass, cabinet, substrate and rock, and a full 200-litre set-up can easily pass 250 kg. Most ground floors cope with small and medium tanks, but on suspended timber floors and upstairs, larger tanks are best placed across the joists and near a load-bearing wall. If you're planning something big or have any doubt, ask a builder or structural engineer before you buy.

Electrical safety

Give every cable a drip loop: let it hang below the socket before rising back up, so drips fall off the lowest point instead of running into the plug. Use a socket protected by an RCD (residual current device), either in your consumer unit or as a plug-in adaptor. Keep extension blocks off the floor and out of the splash zone, and never use equipment with damaged cables.

Your before-you-buy checklist