Buying second-hand aquarium equipment

Goldfish need a great deal of water, and a tank that holds it is expensive new and cheap used, because people leave the hobby with the equipment still in the garage.

That makes the second-hand market the single largest lever on whether a goldfish gets the volume it needs — which is most of what goldfish keeping is. It is also where the riskiest purchase in the hobby sits, and the risk is not the one people expect.

Where the good ones come from

The listing tells you more than the photographs do.

Someone leaving the hobby is the best case, and a common one. The tank has been running, the equipment matches it, the fish are often still in it, and the seller wants it gone in one transaction rather than in parts. Ask the four questions below and you will usually get straight answers, because there is nothing to be gained by anything else.

Someone selling it in pieces is worth more caution. Not dishonesty — it simply means the tank has been broken down, often months ago, so the biology is gone, the history is unverifiable, and the things you most wanted to look at no longer exist.

A house clearance or an inherited setup is the case with no information at all attached to it. Fine for glass and cabinets. Clean and dry everything else thoroughly.

And the source almost nobody uses: a local aquarist society. The rehoming guide makes this argument from the other side — societies are the best route for a fish and the least-used one — and it works identically for equipment. Members are leaving and joining constantly, they know exactly what their tanks had in them, and they will tell you, because the whole point of the club is that they care about it going well.

What is worth buying used, and what is not

Item	Used?	The reason
Glass tank	Yes — the main event	Sound if it passes a fill test; the biggest saving
Stand or cabinet	Yes, with care	Check for water damage and swelling at the base
Filter (external or internal)	Yes, with a decision	Best value and the real risk; see below
Air pump	Usually	Diaphragms perish; cheap to replace, easy to test
Lighting	Yes	Little to go wrong; LEDs dim slowly and visibly
Hoses, pipework, spray bars	Yes	Scrub, rinse and dry; replace anything cracked
Decor and substrate	With caution	Carries pathogens, snails and medication residue
Heater	No	Cheap new, unknowable hours, and it fails dangerously
Test kits	No	Reagents expire, and an expired kit reads confidently wrong

Tanks: test rather than inspect

The tank guide owns this and sets out the checks properly — silicone condition, chips along the bottom edge, deep scratches, and the fill test that actually settles it. Two things worth adding here.

The stand is part of the purchase. A cabinet that has held a leaking tank has swollen chipboard at the base, and it will not carry the weight of a refilled one. Press the plinth and the back panel; anything spongy is finished.

Move it properly. A glass tank is carried under its base by two people, never by the top trim and never slid across a surface. Empty, drained and dry — a tank with water still in the sump of its stand is heavier and much less predictable than it looks.

Filters: the best buy and the riskiest one

Here is the thing that makes this page necessary.

A filter from a running tank arrives with colonised media — the bacterial population that cycling otherwise takes four to eight weeks to build. Seeded properly, a new tank can be ready in days. This site says seed with mature media from a healthy tank in a great many places, and it is genuinely the only shortcut that works.

It also arrives with whatever else lived in that tank: ich and other parasites, bacterial populations, snails and their eggs, and anything the seller was treating for. Media is a sponge in the literal sense.

Those two facts point in opposite directions, and they force a decision that cannot be split.

Taking the media live

Reasonable when you can satisfy yourself the source is healthy:

• The tank is still running and you can see the fish. Five minutes watching is worth more than any answer to a question. Look for the things signs of a healthy goldfish lists — fins carried well, steady breathing, active interest in you — and for the things it does not: white spots, flicking, clamped fins, a fish sitting apart.
• They have been in it a long time. A tank running for years with the same fish is a stable population. One set up three weeks ago tells you nothing.
• Nothing has died recently. Ask directly.
• No recent medication. Covered below.

Then keep it alive on the way home:

Wet, aerated, and used the same day. The bacteria need oxygen and moisture. Bring a bucket, ask for enough of the seller’s own tank water to cover the media, and leave the lid off. Keepers report that media which dries out stops working, and that several hours in a sealed bag of stagnant water is not much better. Run it in your tank as soon as you arrive. Filter media covers where it goes and in what order.

Sterilising instead

If you cannot see the fish, or they are gone, or the answers are vague, treat the filter as a box:

Strip it completely, throw away the media, scrub the body and impeller well, clean and dry it as set out below, and start the cycle from scratch. You have bought a good filter at a good price. You have not bought a head start, and pretending otherwise is how a new tank gets a disease before it has fish in it.

The impeller and its shaft are the parts that actually wear. Spin it by hand: it should turn freely with no grinding and no side-to-side play. Impellers are a cheap spare part, and worth one.

The heater is the exception

Buy it new. The heater buying guide covers what to look for, and the answer is less about wattage than about how low the thermostat will go.

A heater is among the cheapest items in a setup, its remaining life is unknowable, and its dangerous failure is not the obvious one. A heater that fails off gives you a cold tank and time to notice. A heater whose thermostat sticks on keeps heating, and a closed aquarium can reach lethal temperatures within a day — the temperature guide covers why heat is the harder problem for goldfish in the first place, and nothing you can inspect on a used unit tells you how close it is to that.

The same argument applies, less sharply, to anything with a thermostat or a seal under pressure. It does not apply to an air pump, whose failure is simply that it stops.

Decor, substrate and the residue nobody asks about

Second-hand hard decor is fine after a proper clean, and it is where a great deal of pointless expense can be avoided. Two cautions.

Pathogens and hitchhikers travel on it. Snail eggs in particular are almost invisible and survive a casual rinse — snails and hitchhikers covers what arrives that way and why most of it is a nutrient reading rather than an infestation. The wider framing belongs to quarantining a new goldfish, which treats plants, snails and used equipment together as the biosecurity gap that even careful keepers leave open; this page is the purchase end of the same problem.

Medication binds and leaches. Copper-based treatments in particular bind to silicone, porous decor and substrate, and come back out slowly over a long time. Goldfish tolerate traces better than invertebrates do, but a tank that was dosed heavily is worth knowing about — and it is exactly the sort of thing a seller does not think to mention because it was two years ago. Using medications safely covers why residues matter more than people assume.

Check anything sharp with a fingertip before it goes in, the way decorations sets out. A bargain ornament with an edge on it is not a bargain.

The four questions

Ask these, in this order, and the answers settle most of the risk:

1. How long has it been running? Years is reassuring. Weeks is not.
2. Has anything died in it recently, and of what? The follow-up matters as much as the answer: “I don’t know” is honest and is still a reason to sterilise.
3. Has it ever been treated, and with what? Especially anything copper-based.
4. Is it still set up, so I can see the fish?

The fourth is worth the most. A running tank you can watch for five minutes tells you more than the other three answers combined, and a seller who has broken it down the week before is not hiding anything — they are simply no longer able to show you the thing you needed to see.

When the fish come with it

A common situation, and a different transaction from the one this page has been describing: the tank is advertised complete, and complete includes the goldfish.

Treat this as taking on the animals, not as buying equipment with a sweetener. What follows from that:

Count them and size them before you agree. Four adult fancies in a tank that suited them is four adult fancies you now have to house, and the stocking arithmetic does not become more generous because they arrived free. Two single-tails are a pond commitment, not a tank one.

You cannot quarantine them. Quarantine assumes a separate established system, and if these are your first fish then the tank they came in is your only system. That is acceptable — it is how almost everyone’s first goldfish arrive — but it means the whole group is one unit from day one, and any problem in it is a problem for all of them. If you already keep goldfish, the quarantine guide applies with full force and the new tank is where the newcomers stay for a month.

Keep their water. This is the one case where the seller’s tank water is worth taking in quantity — not for the water itself, which is just water, but because moving fish, media and water together means nothing about their conditions changes at once. Move them the way the handling guide sets out, and get the filter running at the other end before anything else is unpacked.

And look at them properly before you say yes. A free fish with a problem is a veterinary commitment, and the kindest moment to decline is before it is in your car.

Disinfecting what you bring home

The Merck Veterinary Manual advises disinfecting aquarium equipment and storing it dry. Bleach soaks are common in the hobby, but a cited strength and contact time are not available here, so none is given. The steps that need no figure:

1. Scrub off the visible growth first. Disinfectant does not penetrate biofilm, so anything you skip here is simply not treated.
2. Rinse genuinely thoroughly. More than feels necessary.
3. Dry completely, for several days. The time is a keepers’ rule of thumb, not a measured figure.
4. Or replace it. Hoses, diaphragms and spray bars are cheap, and an item you cannot get clean is not worth the risk.

Never soap, ever. Residues cling to acrylic, silicone and porous surfaces and end up on gills. The cleaning guide is emphatic about this for the same reason.

And never any of it to media you mean to keep alive. That is not a caution against overdoing it; it is the other half of the decision above.

The stand, the route and the floor

Worth a paragraph because it is where second-hand buying most often goes wrong after the purchase rather than during it.

Measure the route before you agree to collect: doorways, stair turns, and whether the tank passes through on its side. Then check what it is going on and what that is going on — a large used tank is often a large tank, and floor loading is a real consideration above the ground floor. The volume calculator will turn the dimensions into litres, and litres into kilograms is simply the same number.

When second-hand is false economy

Three cases, all of them common:

A tank that is too small. The saving is real and irrelevant. A 60-litre tank at a third of the price is still a 60-litre tank, and the fish it was bought for will outgrow it inside a year. This is an expensive mistake on the list, because the replacement is the whole purchase again.

A filter you end up sterilising. Not a loss — but if the mature media was the reason for the price you were willing to pay, and the source turned out to be unknowable, then you bought a filter body at a filter-body price and should feel fine about it rather than using the media anyway.

Anything you would replace within a year. Air pump diaphragms, hoses, suckers and clips are consumables. Taking them free with a tank is sensible; paying for them is not.

Everything else, the arithmetic strongly favours used, and what keeping goldfish costs sets out why the first year is the expensive one. A second-hand tank and a new heater is the cheapest reliable way to give a goldfish the space it needs, and giving it that space is the decision that matters more than every other one on this site.