Guide
How to acclimate new fish
Float and drip: reduce stress when adding fish to the tank.
- Float first: Float the sealed bag on the tank surface for 15-20 minutes to equalise temperature before doing anything else.
- Drip for sensitive fish: Discus, wild-caught species, and invertebrates need drip acclimation over 45-60 minutes - floating alone isn't enough.
- Never pour store water in: Net or cup the fish across. Store water can carry pathogens and ammonia, and your tank doesn't need either.
- Turn the lights off: A dark tank reduces the territorial aggression that new fish trigger in residents.
- Watch for an ammonia spike: Adding fish to a young or heavily stocked tank can stall the cycle. Test within 24 hours of adding new stock.
- Skip the first feed: Don't feed for at least two to four hours after adding fish. They won't eat anyway, and uneaten food will just foul the water.
Why acclimation matters more than most guides admit
Fish arriving from a store have been living in water that probably differs significantly from yours - in temperature, pH, hardness, and ammonia load. The bag water accumulates ammonia from the fish's own waste during transit, and as ammonia builds up in that low-oxygen environment, the pH often drops, keeping the ammonia less toxic (at low pH, ammonia exists mostly as the less harmful ammonium ion). The moment you equalise temperatures and open the bag, pH can shift fast - and suddenly that dissolved ammonia becomes far more dangerous.
That's the part most guides skip. Acclimation isn't just about temperature shock. It's about managing a cascade of chemical changes so the fish has time to adjust to each one gradually rather than hitting them all at once.
A sudden shift of even 3-4°C (5-7°F) can cause osmotic shock and immune suppression. Some fish die days later from infections triggered by that single stressful event.
Before the fish arrive: set your tank up to receive them
Good acclimation starts before the bag hits your water. Run through this before you leave for the fish store or before an online delivery lands on your doorstep.
- Test your water. Know your actual pH and temperature, not what you assume them to be. If your tank is sitting at pH 7.8 and the store runs pH 7.2, you need a slower drip to buffer that gap.
- Have a clean bucket or container ready - one that has never had soap or cleaning products in it. A dedicated 5-10 litre fishkeeping bucket is worth owning.
- Prepare airline tubing and a small valve or a knot in the tubing for drip acclimation. A dedicated drip acclimation kit costs a few pounds and is worth keeping in a drawer.
- Make sure the tank is fully cycled. Adding fish to an uncycled tank is a separate, larger problem - read the how to cycle a fish tank guide if you're not sure where yours stands.
If you're adding fish to a tank that already has residents, consider whether the current bioload can handle more. A stalled or immature cycle will spike ammonia fast when new fish are added. Use our free stocking calculator to check before you buy.
Step 1: Float the bag
This step is non-negotiable for every fish, every time. Float the sealed bag on the surface of your tank for 15-20 minutes. The goal is simple: get the water inside the bag to match the temperature of your tank water before you open anything.
Turn off the aquarium lights while you do this. A new fish pressed against a bag in full light while your existing fish investigate it through the glass is a stressful way to begin. Lights off and a darkened room settles everyone down.
If the bag is small and your tank has a tight lid, you can float it in a bucket filled with tank water instead - it achieves the same temperature equalisation without the awkward balancing act. For large bags or multiple bags, a cooler box half-filled with tank water works well.
On a cold delivery day, the bag temperature can be 10°C (18°F) below your tank. Give it the full 20 minutes, and if the bag still feels noticeably cool to the touch, give it another five. There's no penalty for going longer on the float.
Step 2: Drip acclimation - when and how to do it
Floating handles temperature. Drip acclimation handles everything else - primarily pH and hardness. Not every fish needs it, but knowing when to use it matters.
Which fish need drip acclimation?
- Invertebrates - always. Shrimp and snails are dramatically more sensitive to water chemistry shifts than fish. Even a modest pH swing can kill a tank of neocaridina shrimp within hours.
- Wild-caught fish. Altum angelfish, wild discus, wild bettas, most wild-caught cichlids - these fish come from very specific water conditions and don't buffer stress well.
- Sensitive species. Discus (Symphysodon spp.), apistogrammas, most corydoras, and many small tetra species benefit from a longer, slower acclimation.
- Expensive fish. Even if the species isn't technically delicate, the financial and ethical cost of losing a £40 fish to a rushed acclimation is reason enough to drip it.
The drip process
After the temperature float, pour the bag contents - water and fish - into your clean bucket. Don't fill the bucket more than a third of the way. Then run a length of airline tubing from your tank down into the bucket. Tie a loose knot in the tubing or use a small flow-control valve to restrict the flow to roughly 2-3 drips per second. Start the siphon by sucking the end briefly (or use an air pump and a gang valve), then clip the tube to the side of the bucket and leave it to run.
Over 45-60 minutes, the bucket water will gradually shift toward your tank parameters as tank water slowly dilutes the store water. When the bucket volume has roughly doubled, the fish are ready to move. For invertebrates or particularly sensitive wild-caught fish, some aquarists run the drip for 90 minutes - there's no harm in going longer.
Most people set the drip rate too fast. Four to six drips per second feels slow, but at that rate you're tripling the bucket volume in 20 minutes and barely giving the fish time to adjust. Two to three drips per second over 45-60 minutes is what you're aiming for. Slow is the point.
Step 3: Moving the fish across
When acclimation is done, use a soft net or a small cup to transfer the fish from the bucket into the tank. Never pour the bucket or bag water into your tank. That water contains store-tank bacteria, potentially elevated ammonia, and possibly parasites or pathogens you don't want introduced.
Work quickly at this stage. The fish are out of water for as little time as possible. Have the tank lid open and ready before you pick up the net.
For fish that are difficult to net - spiny species like puffers, or fish that wedge themselves into corners - use a small plastic cup or container to scoop them up with minimal water and transfer them that way. Chasing a stressed fish around a bucket with a net for five minutes defeats the purpose of the careful acclimation you just did.
The fish store will sometimes tell you to add a cup of store water to your tank to help the fish "recognise" the new environment. This is not a real thing. Fish don't navigate by sniffing their old water. What you do get from adding store water is exposure to whatever bacterial load or parasites were in that system. Don't do it.
Step 4: The first 24 hours
New fish almost always hide for the first few hours. This is normal. Resist the urge to rearrange decor to find them, and don't add food for at least two to four hours. Skipping the first feed has two benefits: the fish are too stressed to eat anyway, and uneaten food sits and rots while your tank's biofilter is already dealing with additional waste from the new fish.
Keep the lights low or off for the first evening. If you have aggressive tank residents, watch for territorial behaviour - new fish can't escape to familiar hiding spots and are at a disadvantage in the first hours.
Test your water at the 24-hour mark, particularly ammonia and nitrite. Adding fish increases the biological load on your filter, and if the colony of beneficial bacteria hasn't caught up yet, you'll see a spike. If ammonia climbs above 0.25 ppm, do a partial water change of 25-30%. A regular water change schedule matters too - see how often to do water changes for guidance.
Log your parameters immediately after adding new fish and again at 24 hours. App-aquatic lets you track test results over time, so you can spot a rising ammonia trend before it becomes a crisis - especially useful in the weeks after adding new stock.
What most guides miss
Quarantine is the step that actually saves fish
Acclimation gets all the attention, but quarantine is where experienced fishkeepers protect their tanks. Running new fish through a separate quarantine tank for 2-4 weeks lets you observe them for disease before they're in contact with your existing stock. Ich, velvet, internal parasites, and bacterial infections frequently don't show symptoms at the store. You won't see them during a 15-minute acclimation. But you will see them two weeks into quarantine.
A quarantine tank doesn't have to be elaborate - a 20-40 litre bare-bottom tank with a sponge filter and a heater is enough. The sponge filter can run permanently in your main tank to keep it seeded with bacteria, ready when you need it.
The pH gap problem
If your tank water pH and the store water pH differ by more than 0.5, take the drip acclimation slower than the standard recommendation. A difference of 1.0 or more in pH is exponentially more stressful than that number suggests, because the pH scale is logarithmic. Consider running the drip for 90 minutes and reducing the drip rate slightly. Understanding the full picture of aquarium water parameters - including how pH, KH, and hardness interact - is worth doing before you buy any fish that came from significantly different source water.
Don't acclimate in the store bag longer than two hours
The oxygen in a sealed store bag runs out. Two hours from the moment you bought the fish is about the outer limit for most tropical species. If you're combining a long drive with a detour, get the fish home first.
In summer, a car parked in the sun can heat a bag of fish to lethal temperatures in under 30 minutes. Keep bags in a cool bag or cooler box during transport, and never leave them in a hot car.
Acclimating to harder or softer water than the store
Most fishkeepers assume that as long as the target parameters are within the species' acceptable range, acclimation will handle the transition. That's mostly true, but if you're keeping soft-water species like cardinal tetras (Paracheirodon axelrodi) from a store with hard water - or vice versa - the osmotic difference is significant. Go slower on the drip. Also consider whether the fish you're buying have actually been kept in appropriate conditions; a discus maintained at pH 7.8 and 300 ppm hardness at a general pet store is going to be stressed before it even reaches you. Check best fish for beginners if you're still building your stocking list and want species that handle typical tap water more forgivingly.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow long should I float the bag before opening it?
15-20 minutes is the minimum for temperature equalisation. If the bag feels noticeably cooler than your tank after 20 minutes, give it another 5-10. There's no downside to going longer, as long as you're not leaving the fish in a sealed bag for more than two hours total from purchase.
Do I need to drip acclimate every fish?
No. Hardy, commonly sold species - zebra danios, guppies, platies, most goldfish - do fine with the float method alone. Drip acclimation is worth the extra time for invertebrates (always), wild-caught fish, discus, and any fish that was clearly being kept in very different water conditions at the store.
Can I add fish straight to the tank from the bag if the water looks clear?
No. Water clarity tells you nothing about temperature, pH, or ammonia. Even if the store water looks clean and your tank water looks clean, the parameters can differ enough to cause osmotic shock. Always float at minimum.
Why is my new fish hiding and not eating?
This is completely normal for the first 24-72 hours, and sometimes longer for shy species. The fish is orienting to new surroundings, finding safe spots, and assessing the other tank inhabitants. Don't chase it out of hiding, don't overfeed trying to entice it, and give it time. If it hasn't eaten after a week, that's when to investigate further.
Should I turn off the filter when adding new fish?
No. Keep the filter running. You might briefly reduce flow if your new fish are very small and risk being pulled toward the intake, but switching off the filter removes the oxygenation and biological filtration your tank needs. If you're using a drip bucket, make sure it's positioned away from strong surface flow so the drip line doesn't get disrupted.
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