How to add fish to your fish tank

How do I add fish to my fish tank?

Adding new fish to your fish tank isn’t simply a case of throwing them into the tank straight out of the bag. It is important to gradually introduce them to the aquarium using a process called acclimatisation. Here we explain what acclimatisation is and show you step by step how to add fish to your aquarium safely.

What is acclimatisation?

Acclimatisation is the process of introducing fish to a new environment without causing stress or any sudden changes. Acclimatisation is usually required when purchasing fish from a retailer and bringing them home. Although fish also need to be acclimatised if they’re being transferred from one fish tank to another in your home. The process doesn’t take long, and your fish can usually be introduced safely within the space of an hour, but it may take a few weeks for your fish to properly adjust to their new surroundings.

Why do fish need to be acclimatised?

When introducing fish to a new environment, any sudden changes can send them into a state of shock and weaken their immune systems. In a worst case scenario the changes can even kill your fish. The various properties found in water is partly responsible for this, along with the temperature of the water. pH levels, salinity and hardness (the amount of dissolved minerals) vary from tank to tank, meaning that water in a new aquarium will take some getting used to.

Therefore, to acclimatise fish properly, a few steps need to be taken to make a warm welcome for the little guys.

Before acclimatising new fish

Before you go to the aquatic shop to choose your fish, it can be a good idea to use a fish tank test kit to check your water quality. If the ammonia in your fish tank is quite high, it is worth waiting a week or two until this problem has been solved.  Transporting fish to a new aquarium can be stressful enough and if you add ammonia or nitrite to the mix this will only make the problem worse.

If you already have fish in your tank, it can also be a good idea to do a quick gravel clean and partial water change within the 24 hours before you purchase the fish. Make sure that you add the appropriate amount of dechlorinator to the new water to make it safe for fish.

Buying new fish for your aquarium

When looking around the aquatic shop, look at the conditions that the fish are kept in. It is common for aquatic shops to keep their fish in more crowded conditions than is advisable, due to the rate that the fish reproduce when kept in small tanks. If you see any dead fish in a tank, this can be a sign that they are not being looked after properly if they look as though they have been dead for a while. Obviously fish dying is inevitable at some point, particularly when so many are kept together. But in order to prevent the spread of disease and keep ammonia levels low, dead fish need to be removed from an aquarium as soon as possible.

Ask members of staff for advice. They should have sufficient knowledge and will be able to advise you which fish can be kept with one another and which fish are hardy enough to survive the nitrogen cycle if your fish tank is new. It is best to do a bit of research before you visit the shop though, just in case the staff don’t know as much as they make out. Some stores insist on selling fish in pairs or groups which are not appropriate for their species. Guppies for example should be sold one male for every two females, but you might find some shops which sell them in pairs, one of each sex. This can be bad for the females as they are likely to be harassed and chased by the males.

When you first buy the fish, they should be placed in strong bags ready for transport. Sometimes you might find that they have been double-bagged. This is great for preventing leaks, but once you get them home it can help to remove one of the bags for the acclimatisation process. This can prevent the fish from getting stuck in the bag as you pour the water into the tank.

How to add fish to your fish tank

Before adding the fish to the aquarium, keep the lights off in the tank. If possible, keep lighting in the room dimmed also.

  • Float the bag in the aquarium for about 20 minutes. If it has taken a while to transport the fish to your home it can be a good idea to open the bags to let some air in.  The temperature should now be roughly the same as the water in the aquarium.


  • If you haven’t already, roll down the sides of the bag and add a little bit of aquarium water into the bag to introduce the fish to the pH levels, salinity and hardness without shocking them. After another 20 minutes repeat this process once more.
  • After completing the above, it is time to add the fish to the aquarium properly. It is usually advised that you use a fish net to transfer the fish into the tank. On rare occasions the water from the fish shop could be harmful for your aquarium and so a net will ensure minimal water is transferred and decrease the chance of diseases being spread.


  • The picture above shows one way of moving the fish from the bag to the net, but it can be tricky to do it this way without letting water in the aquarium. A better method is to hold your bag and net over a bucket and pour the water through the net, before moving the fish from the net into the tank.


  • Some people prefer to simply turn the bag on its side and allow the fish to swim into the fish tank, rather than using a net. This could be described as a more natural approach and less stressful for the fish.

After you’ve added the fish

Now that your fish have been introduced, you might feel like feeding them. It is best to avoid feeding for at least 24 hours as they’re likely to be too stressed to eat, resulting in extra waste. However, if you’ve had a group of other fish in the fish tank that you’ve had for a while you might want to put a bit of food in for them to eat.

Ten Tips

1) Turn the lights off when acclimatising fish and keep them off for several hours afterwards so not to startle the fish.

2) Never add too many fish at once. Fish numbers all depend on the size of your fish tank.

3) Do not feed your fish for the first 24 hours. When you do start to feed the fish, give them a tiny amount at a time. They’ll soon get in the habit of feeding in their new environment but it might take them some getting used to.

4) For the first few days, avoid any loud noises or sudden light changes to prevent your fish from getting startled. Don’t add new fish to your aquarium if you’re having a party the next day for example. The little guys are likely to be nervous at first and so it’s best to keep loud noise to a minimum at first.

5) Remember to do a little research before buying your fish to make sure that they are compatible with one another.

6) If you’ve recently had problems with ammonia, nitrite or diseases in your aquarium, hold off buying any new fish for a few weeks until the problem has been resolved and the tank has been running smoothly for a while.

7) Don’t add any new fish if you’ve had problems with your filter or heater. Wait until everything is running smoothly.

8) If you already have fish in your aquarium, give your tank a good clean the day before you add your new fish. No need to go overboard as you still want beneficial bacteria to remain in the water. A quick gravel clean, soak of the filter media and water change should do the trick.

9) If your fish tank is new, remember that location is important. Avoid placing it next to sources of heat such as radiators or too close to a window. Also avoid TVs and radios.

10) Test the water regularly to ensure water quality is up to scratch.

Jenni’s aquarium

Swell content writer, Jenni Hill set up her first aquarium two months ago. Here she writes about acclimatising new fish.

One of the most common mistakes people make when acclimatising new fish to an aquarium is adding too many at once or overpopulating the fish tank.

When I first set up my aquarium I waited a week before adding four cherry barbs. A hardy species of fish, cherry barbs are able to handle increasing ammonia levels in an aquarium and are great for kick starting the nitrogen cycle. After 7 weeks of water changes, test kits and treatments, I added three guppies too.

Guppies are known for being prolific breeders and it is usually advised that you keep two female guppies for every male. This is because the males are frisky little creatures and are known to chase the females around the tank. The males are more colourful than the females and show off their fancy colours in an attempt to ‘woo’ the females and encourage them to breed. The females are fussy though and as much as the boys may try to turn on the charm, the girls often reject their advances. So it is best to have more females so that they can stick together and so that the male attention is spread between them. Another option is to opt for a male only tank. For more information about guppies, here’s our beginners guide to keeping guppies. When in the aquatic shop I told the assistant that I wanted three males but to my surprise he explained that they only sell certain breeds of guppies in pairs – one male, one female. I pointed out that I’d heard the males chase the women but the member of staff was adamant that this is not the case with guppies. Very frustrating! I had my heart set on these pretty little fish so I agreed to take a pair along with another female guppy of a different breed.

A fortnight later, I bought two more fish, this time a black molly and a platy. Mollies are peaceful fish which are great at adapting to both freshwater and saltwater aquariums. Platies are also peaceful and easy to care for and therefore great additions to a community.

Acclimitising them to the fish tank was easy to do. I first floated the bag in the water for about 20 minutes.


This gave the water inside the bag chance to reach the same sort of temperatures as the water in the rest of the fish tank. The cherry barbs and guppies seemed very intrigued about the new additions and couldn’t seem to understand why they could not get through to them.



Next, I opened the bag and used a small clean cup to add some aquarium water into the bag. Never use cups, buckets or jugs that have been cleaned with cleaning products such as washing up liquid or bleach. The slightest trace can harm your fish.



I waited another 20 minutes before adding a little more aquarium water to the bag. After an hour had passed in total, I rolled down the sides of the bag and used a net to transfer the fish into the tank. Some people choose to tip the bag on its side and allow the fish to swim into the aquarium themselves because they believe it is less stressful. It’s all a matter of personal preference really.



  • Phillip Connor Posted 19/12/2017 at 2:03 pm | Permalink | Reply

    Hiya Guys

    just a wee quick question.

    I bought a 200L tank second hand and when I bought it there was dead fish in the bottom of the tank. It had stones and fake plants. The water that was in the tank was black with dirt. I emptied the tank and removed all the plants and stones. I washed the tank with boiling hot water and filled it up again. I let the tank run for 2 weeks and started adding my fish slowly. I then noticed that the fish were all starting to die. we had 20 guppies and 12 shrimp in the tank now only one guppy. I took the remaining fish out an put him into another tank that I have up and running for mounts. I have removed all the water from the 200L tank.

    I just want to know how to set the tank up in the correct manner? Do I start the whole process again and hope all will be okay or what is the best course of action to take.

    I would be really greatfull if you could answer.

    Many thanks

    Phillip Connor

    • Fran Marshall Posted 20/02/2018 at 8:42 am | Permalink

      Hi Phillip,

      Sorry for the delay in getting back to you, due to maternity leave. How did you fare with setting the tank up again? I would say the key to success is slow and steady – 20 guppies is a lot, but this depends on the time frame they were put into the tank. A brand new tank with 2 fish going into it is increasing its work load by 200%, regardless of the size. If you then add another 2 fish, you’ve increased the load by 100% again, and so on. Generally as a rule, try to only had no more than half the fish in the tank – if you have 6 small guppies, and no more than 3, but next time you could add more. Also, guppies are very sensitive little fish, and not ideal for a new set up, but best when a tank is matured through. Similarly with shrimp – any alterations in water quality and inverts will always be the first to suffer.

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