Transporting a fish safely: the bag, the journey and the arrival

Quick answer

A settled fish in its own tank. Everything below is about getting it to a different tank without the journey being what kills it.

Fish do not get travel sickness. They have no mechanism for it. What harms a transported fish is oxygen running out, ammonia turning toxic the moment the bag is opened, temperature swinging in a small volume of water, physical damage from thrashing, and a stress response that shows up as disease three to seven days later.

All five are preventable, and most of the prevention happens before the fish goes in the bag.

• Fast the fish for a day or two beforehand. Less food in equals less ammonia in the bag.
• Fill the bag roughly one third with tank water and two thirds with air. The air is not empty space, it is the oxygen supply.
• Keep the bag sealed, dark, upright and insulated, and do not open it to check.
• Never tip transport water into the destination tank.
• Watch for ich, clamped fins and fin rot for a week to ten days after arrival. Transport stress is the trigger, not the destination.

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Never open a transport bag and then leave the fish sitting in it.

In a sealed bag, carbon dioxide from the fish's own respiration builds up and pushes the water's pH down. At that low pH almost all the accumulated ammonia sits in its ionised, comparatively harmless form. Open the bag and the carbon dioxide gasses off within minutes, the pH climbs back, and that ammonia converts to free ammonia, which burns gill tissue.

This is why fish sometimes survive a long journey perfectly well and then die during a slow, careful acclimation. If a bag has been sealed for hours, match the temperature with the bag still closed, then net the fish out promptly.
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Decide in 60 seconds

Why a fish in a bag is in trouble, and from what

Oxygen.

A sealed bag contains a fixed amount of oxygen, some dissolved in the water and much more in the air above it. The fish draws on it continuously and there is no resupply.

Two things accelerate the countdown. Warm water holds less dissolved oxygen than cool water, and warm fish have a higher metabolic rate and use it faster. A hot car therefore attacks the oxygen supply from both ends simultaneously, which is why summer journeys kill fish that would have been fine in March.

The air space is the reserve. A bag that is three quarters water and a quarter air looks kinder and is worse.

The ammonia flip.

Covered above, and the single most misunderstood part of moving fish. The practical rules that come out of it: fast the fish first so there is less ammonia to accumulate, use an ammonia-binding water conditioner for anything longer than a short hop, keep the bag closed, and choose your acclimation method by how long the bag has been sealed rather than by habit.

Temperature.

A litre or two of water tracks the temperature around it quickly. A bag on a car seat in sunlight can climb dangerously in half an hour, and a bag in an unheated boot in winter can fall just as fast. The fish has no behavioural escape.

Physical injury.

A frightened fish thrashes. In a confined bag that produces split fins, scraped scales, damaged barbels and eye injuries, and long-finned fish suffer most because there is more to catch and tear. Some stressed fish bite their own trailing fins in a bag.

Spiny species are a separate problem: catfish, plecos, some loaches and many cichlids lock their fin spines outward when handled, and those spines puncture bags and catch in net mesh so badly that the fish is injured getting free.

Stress, and what it does afterwards.

Handling, confinement and poor water trigger a stress hormone response that suppresses immune function and disrupts the slime coat, which is the fish's first physical barrier. Parasites and bacteria that the fish was previously holding in check take hold.

The result is the classic pattern of a fish that arrives looking fine and breaks out in white spot four days later. Owners blame the new tank. The cause was the journey.

Before the journey

Conditioner, clean tank water, a soft net and a container. Get all of it out and ready before the fish is caught, not while it waits in a bag.

Fast the fish.

A day or two without food for most adult community fish, less for very small or very young ones, and take advice for fry and for species with high metabolic demands. A fish with an empty gut produces far less ammonia and far less waste in the bag. This one step does more than any product.

Do a water change the day before.

The water you bag the fish in comes out of the tank, so make it clean water. A change the day before, not on the morning of, so the tank is settled.

Prepare the destination first.

The receiving tank should be running, heated to the right temperature, cycled, and with the lights off. Arriving with a fish and then setting up is how fish end up sitting in bags.

Assemble the kit.

Fish bags with rounded corners, or square bags with the corners taped across. Rubber bands. A rigid lidded container for spiny or large fish. An insulated box or cool box. A towel. A thermometer. Water conditioner and an ammonia binder. A battery air pump for long road journeys. A soft, fine-mesh net, and a clear container for anything you should not net.

Catch the fish properly.

Two hands, or a net and a container. Guide the fish into a submerged container rather than lifting it through the air in a net wherever you can. A fish lifted in a net loses slime coat against the mesh and panics against it.

Never net a spiny catfish or pleco. Use a container, or a bag held open underwater.

The bag, and how to fill it

One third water, two thirds air.

Enough water to cover the fish comfortably and let it stay upright, and as much air above it as the bag will take.

Use tank water.

Never fresh tap water, even conditioned. The fish is already adapted to its own water chemistry and there is no reason to change two variables at once.

One fish per bag.

Certainly for anything territorial, anything spiny, and anything large. Small shoaling fish can share, but crowding is the fastest route to oxygen depletion.

Double bag anything with spines, and seal the inner and outer bags separately so a puncture in one does not empty both.

Seal with a rubber band, with air trapped inside.

Twist the neck, fold it over, band it. No gap, and no attempt to squeeze the air out.

Bag into a box.

Darkness reduces activity, which reduces oxygen consumption, and it markedly reduces stress. An insulated box does the temperature job at the same time. Wedge the bags upright with towels so they cannot roll.

Long journeys.

Bags filled with pure oxygen rather than air, or breather bags, which allow gas exchange through the bag wall and are designed for exactly this. Add an ammonia binder. For anything over a day, seriously consider a courier that specialises in livestock rather than doing it yourself.

The journey

Travel in the passenger compartment, not the boot. Boots are hot in summer, cold in winter and badly ventilated all year.

Do not open the bag. Not to check, not to let air in, not for reassurance. Every one of those makes it worse, for the reasons set out above.

Do not stop for anything you can avoid. Time in the bag is the variable you control.

Keep the box out of direct sunlight, and set the car's climate control to what the fish needs rather than what you would like.

Drive smoothly. Sudden braking throws a bag around and it is the fish that absorbs it.

If a journey is unavoidably long and you have a battery air pump, air the water through an airline in a rigid lidded container rather than opening and resealing bags repeatedly.

Arrival

Lights off.

The destination tank stays dark for the rest of the day. This lowers stress and, if there are existing residents, reduces the attention a newcomer attracts.

Short journey, under an hour or two.

Float the sealed bag in the destination tank for fifteen to twenty minutes to equalise temperature. Then open it, add a small amount of tank water every five minutes for twenty to thirty minutes, then net or lift the fish out into the tank.

Long journey, several hours or more.

Float the sealed bag to equalise temperature. Then open it and get the fish out promptly. Do not spend an hour dripping tank water into water that has been accumulating ammonia all day, because you have just raised the pH and made that ammonia toxic.

Never tip the transport water into the tank.

It goes down the sink. It carries ammonia, waste, and whatever came with the fish.

Do not feed for the first day.

The gut has been shut down and the fish is not ready. Feeding into a stressed fish fouls the water and achieves nothing.

Then watch, for a week to ten days.

Clamped fins, white spots, cotton-like patches, ragged fin edges, flashing against decor, hanging near the surface or in a corner, refusing food. Quarantine, if the fish is going into an established tank, is what turns one sick fish into one sick fish rather than a whole tank.

What changes by fish

Long finnage is the tissue that tears in a bag. Long-finned fish need a bigger bag, a shorter journey and a closer look at the fin edges on arrival.

Long-finned fish.

Bettas, veiltail and other fancy goldfish, angelfish, long-finned tetras and barbs. The fins split and tear against the bag, and a stressed fish may bite its own. Use a noticeably larger bag than the fish's size suggests, keep the journey short, and inspect fin edges for the following week, because a torn fin is the entry point for fin rot.

Deep-bodied and large fish.

A fancy goldfish is both long-finned and a heavy oxygen user, which makes it one of the harder fish to move well.

Goldfish, koi, oscars and other large-bodied fish use a great deal of oxygen for their size. Give them far more air space, fewer fish per container, and a rigid lidded tub rather than a bag once they are past a certain size.

Spiny species.

Catfish, plecos, loaches and many cichlids. Never net them. Use a container or a bag held open under the water. Double bag them.

Labyrinth fish.

Bettas, gouramis and their relatives breathe atmospheric air at the surface as well as through the gills. They must be able to reach the surface inside the bag, which means the bag must never be filled to the top. This is one of the few cases where too much water is immediately lethal rather than gradually harmful.

Fry and very small fish.

They get trapped in bag corners and in the surface film. Use rounded or taped bags and keep the numbers low.

Marine fish and invertebrates.

Less forgiving on both temperature and salinity, and corals and many invertebrates must never be exposed to air. Take specialist advice rather than adapting freshwater practice.

Season and climate

Hot weather.

The dominant risk. Pre-cool the car before the fish goes in. Use a cool box with a cool pack wrapped in a towel and never in contact with the bag, because direct contact chills one side of a small water volume unevenly. Travel early or late in the day. Never leave the box in a parked car, even briefly, even in shade.

Freezing weather.

Pre-warm the car. Use a shipping heat pack of the type sold for livestock, wrapped, positioned so it warms the air in the box rather than touching the bag. Never a hot water bottle or a heated seat. Check the destination tank has not been cooled by the house being empty.

Both.

Insulation cuts in both directions and costs nothing. A polystyrene box, or a cool box with a towel, is the single most useful piece of equipment for moving fish in any weather.

A note on moving house.

Moving one fish and moving a whole aquarium are different problems. The fish are the straightforward part. Keeping the filter's bacterial population alive through the move, so the tank does not restart its cycle at the far end with fish already in it, is the difficult part, and it is worth planning separately.

What never to do

Do not transport a fish in an open bucket. It splashes, it cools, and fish jump.

Do not use a supermarket carrier bag. Soft plastic, sharp corners, and possible chemical residue.

Do not bag a fish in fresh tap water.

Do not open the bag during the journey.

Do not squeeze the air out of the bag before sealing it.

Do not leave the box in a parked car in any weather.

Do not put a heat pack or a cool pack in direct contact with the bag.

Do not net a spiny catfish or pleco.

Do not drip-acclimate a bag that has been sealed for many hours.

Do not add transport water to the tank.

Do not move a fish that is already gasping or listing without asking a vet first, and do not assume the move is the only option, because a water sample and a video often answer the question without the fish leaving home.

When to get help

Contact an aquatic or exotics veterinarian if a fish arrives gasping at the surface, lying on its side, unable to stay upright, or bleeding, and if it does not settle within a few hours of being in good water.

Contact them in the following week for white spots, cotton-like growths, ragged or receding fin edges, flashing against decor, refusal to eat beyond the first day, or any breathing change.

Get advice before transporting a fish that is already unwell. For many problems a water sample in a sealed clean container, your test results and a short video of the fish swimming give an aquatic vet more than the fish itself would, and the journey is a genuine risk to an already compromised animal.

Bring these if you do go in: how long the journey took, what the water in the bag was, whether the bag was opened, what temperature the water was at each end, what the fish was fasted for, your destination tank's ammonia, nitrite, nitrate, pH, hardness and temperature, and the before photograph.

Expect the questions to be about the journey and the water rather than about the fish. In aquatic medicine the history of the water is the history of the patient, and a transport failure leaves a signature that a good clinician can read from those numbers alone.