Taking on a rehomed fish or an inherited tank

Quick answer

Taking on somebody else's fish is not like taking on somebody else's cat. You are inheriting three things: an animal of unknown age and unknown history, a living bacterial filter you cannot see and can kill in an afternoon, and a body of water whose chemistry the fish has spent years adapting to.

The most common way this goes wrong is kindness. The new owner gives the tank a proper clean, replaces the tired-looking gravel, rinses the filter sponge under the tap, and does a big water change to freshen things up. Every one of those actions is reasonable, and together they can kill a fish that had been surviving quite happily in conditions that looked terrible.

The setup you are offered is often the reason the fish needs rehoming. Plan the tank the fish should be in, not the one it arrives in.

• Never rinse filter media under a tap. Chlorine kills the bacteria that keep the fish alive.
• Do not do a large water change on an inherited tank. Bring the parameters round over weeks, not in one session.
• Find out the adult size and lifespan of the species before you agree to take it. Many are decades and many are large.
• Fish do not grow to the size of their tank. They become deformed and short-lived instead.
• Never release an unwanted fish into a pond, river or lake. It is illegal in many places and it is a disaster for both the fish and the waterway.

Decide in 60 seconds

What you are actually inheriting

A bacterial filter.

Fish excrete ammonia, which is toxic. In a working aquarium, one group of bacteria converts ammonia to nitrite, which is also toxic, and another converts nitrite to nitrate, which is much less so and is removed by water changes and plants.

Those bacteria live on surfaces: filter sponges, ceramic media, the substrate, the glass and the decor. They are not in the water. This single fact drives most of what follows.

If you keep the media wet, in old tank water, and get it running again quickly, you keep the colony. If you rinse it under a tap, let it dry out, seal it in a bag for a day, or throw it away and buy new media, you have removed the tank's ability to process waste, and the fish will be exposed to ammonia within days.

Water the fish is adapted to.

Fish osmoregulate constantly against the water they are in, and they adjust to gradual change. A fish that has spent three years in hard, nitrate-heavy, acidified water is physiologically set up for that. Bringing it into pristine water instantly is a shock, not a rescue.

An unknown health history.

You will rarely be told about a previous outbreak, an old medication course, or a fish that died last month. Assume the worst and quarantine any individual fish before it meets yours.

Equipment of unknown age.

Heaters fail in the on position and cook a tank. Filters fail in the off position. Old silicone seams leak once a tank is refilled after years dry. Test the tank filled, outside or in a bath, before it stands on a floor or a piece of furniture.

A weight problem.

Water is heavy, and a filled aquarium with substrate is much heavier than people expect. Check that the floor, the stand and the surface are all up to it, and that the stand is genuinely level, because an unlevel tank stresses the seams.

The reality check on size and lifespan

The honest part of this conversation is that most rehomed fish are being rehomed because somebody bought an animal without knowing what it would become.

Common and comet goldfish are long-lived, grow substantially, and produce a lot of waste. They belong in a large tank or a pond, not in a bowl or a desktop nano tank.

Koi are pond fish for life and grow to a size no indoor aquarium can accommodate.

Common plecs are sold as small algae eaters and grow into large, heavily built catfish that produce far more waste than they ever remove as algae.

Oscars, pacus, shark catfish and several other species sold as attractive juveniles reach sizes that no ordinary home aquarium can hold.

Before you agree to take a fish, find out its actual species, its adult size and its lifespan, and then look at your tank honestly. It is far kinder to say no now and help find an appropriate home than to accept and then rehome again in two years.

Fish do not grow to the size of their tank.

This is the most persistent myth in the hobby and it is worth killing properly. In a tank that is too small or poorly maintained, the external growth of a fish slows while the internal organs continue to develop. The result is a stunted animal with a compressed body, a disproportionately large head and eyes, spinal curvature, organ compression and a much shorter life. A small fish in a small tank is not evidence that the arrangement is working.

Assessing the fish before you commit

Look for a body that is smoothly filled out along the back and belly. A dorsal ridge that has become a sharp edge, or a hollow belly, means the fish has been underfed or is unwell.

Look at the spine from above and from the side. Curvature acquired during growth does not straighten later.

Look at the gill covers. A fish that grew up in poor water often has shortened or missing gill covers, leaving the gill tissue exposed. It is permanent and it makes the fish more vulnerable for the rest of its life.

Watch the gills move. Rapid, forced gill movement, or gills that look pale or unusually red when you glimpse them, points to gill damage from ammonia, nitrite or parasites.

Look at the fins. Ragged, eroded or blood-streaked edges suggest bacterial infection or chronically poor water. Fins held tight against the body indicate a fish that feels unwell.

Look at the eyes. Cloudiness, bulging or a sunken appearance all mean something.

Watch it swim. Struggling to stay down, sitting on the bottom, listing, or hanging at the surface all need explaining before you take it on.

Ask the owner directly what the water is like, whether they test it, when they last changed it, what the fish is fed and how often, and whether anything has died recently. The answers, including the vague ones, are information.

Moving it without killing the filter

Plan the whole move as one trip.

Decide where the tank is going and prepare that spot before you leave.

Bag or bucket the fish in its own tank water, with enough air space above, kept dark and reasonably warm. A cool box works well for temperature stability. Do not feed for a day beforehand, which reduces waste in transit.

Keep as much of the old water as you practically can, in clean sealed containers.

Put the filter media into a container of old tank water, sealed but not squeezed. It must stay wet and it must not sit for days. The bacteria need oxygen, so a long journey in a tightly sealed bag is not ideal.

Move the substrate wet, in old tank water, if you are taking it. Substrate that has gone anoxic in a bucket smells of rotten eggs and should not go back into the tank.

Set up at the other end straight away. Substrate in, old water in, top up with dechlorinated water at a matched temperature, filter running, heater on, then acclimatise and add the fish.

Never rinse anything in tap water. If media needs cleaning, squeeze it gently in a bucket of old tank water.

Everything for the move ready in one place: a bucket of old tank water, a net, dechlorinator and an air supply. The filter media travels wet, in tank water, and goes back on line immediately.

The first fortnight

Test ammonia, nitrite, nitrate and pH daily, using a liquid test kit rather than strips. Write the numbers down.

Expect a wobble. Even a well-handled move disturbs the bacterial colony, and a small ammonia or nitrite spike in the first week is common. Respond with small partial water changes using dechlorinated water at the right temperature, not with a full clean.

Feed lightly. Uneaten food is the fastest way to make a disturbed filter worse. A fish can comfortably go a few days on very little while the system settles.

Bring the chemistry round slowly. If nitrate is very high, or the pH has drifted far from where you want it, correct it over weeks with small frequent water changes rather than in one large swing.

Leave the deep clean for later. Once the parameters are stable and the fish is settled, you can start improving the substrate and decor gradually, a section at a time, so the bacterial colony is never removed all at once.

Watch behaviour rather than only the numbers. A fish that starts exploring, eating readily and holding its fins open is settling. A fish that hangs at the surface, gasps, sits on the bottom or flicks against objects is telling you the water is wrong.

Ageing fish, honestly

There is no reliable way to determine the age of most aquarium fish from the outside, and anyone who tells you otherwise is guessing.

What you can observe is decline. Older fish commonly fade in colour, become less active, spend more time resting, respond more slowly, become thinner along the back, and may develop cloudiness in the eyes. Appetite drops and recovery from any setback takes longer.

The useful implication is not a number. It is that a fish acquired as an adult of unknown age may have less time than you expect, that changes should be made gently, and that a slow decline in an older fish is not automatically a disease to be medicated.

What never to do

Never release an unwanted fish into a pond, river, canal or lake. It is illegal in many jurisdictions, released fish introduce disease and parasites to native populations, and species that do survive become invasive. The fish itself usually dies slowly.

Never rinse filter media, sponges or ceramic rings under a tap.

Never replace all the substrate, all the media and all the water in one session.

Never add medications the previous owner hands over. Treating without a diagnosis is how tanks develop resistant infections and how invertebrates are lost to copper residues that remain in silicone and substrate for a long time.

Never use aquarium salt as a routine tonic. It is a treatment with specific uses, and many species tolerate it poorly.

Never keep a betta or any tropical fish unheated or in a bowl, however it arrived.

Never assume a stunted fish is a small fish. It is a large fish that has been prevented from growing properly.

When to get help

Contact an aquatic specialist or a vet who sees fish urgently if the fish is gasping at the surface, lying on its side, unable to stay off the bottom, bleeding at the fins or gills, or if ammonia or nitrite is detectable in the water.

Ask for advice within a few days for a fish that will not eat after the first few days in a new home, for persistent flicking or rubbing against surfaces, for white spots, fuzzy patches, ulcers or reddened areas, and for any tank where the pH has moved sharply.

Ask before you commit if you are unsure of the species, the adult size, or whether your tank can house it. That conversation is far more useful before the fish is in your car than after.

A settled fish holds its fins open, uses the whole tank, feeds readily and breathes evenly. That is the target for the end of the first fortnight.

The point of the exercise: the same animal, in a tank that matches what it will become rather than what it was sold as.