Things to think about before you buy new fish

Aquarium keeping can be an interesting hobby. Buying new fish is all part of the fun and introducing them to your aquarium for the first time can be an exciting process. When setting up a new aquarium, it is widely advised that you only add a few fish at a time rather than filling the fish tank full of life as soon as it’s taken out of the box and filled it with water. If you have followed the above advice, then you’re off to a good start. But after the initial start up and after adding your first few fish a week later, how long should you wait before you add more? Here we explain some of the things to look for before introducing more fish to your aquarium.

How big is your fish tank?

The first question to ask yourself is whether your fish tank is big enough to allow the fish you introduce plenty of room. An overstocked and crowded tank is a recipe for disaster, and your fish are unlikely to survive for long. If you own a tropical tank, it is widely advised that you try to stick to no more than one cm of fish per litre of water. This should give your tank inhabitants lots of space to swim. Marine fish often require much more space and so we recommend researching the individual species to find out more.  Overstocking an aquarium not only limits space, but it will also get polluted quicker. The more fish you include, the more waste will be dispensed into the water, meaning that you will have to conduct more water changes in order to prevent your fish from getting ill.

It is generally advised that you do a 20% weekly water change using a gravel cleaner in order to dilute the water and remove waste. If you have a very small tank then it may help to up the changes to twice a week. You’ll soon learn what works best for your aquarium over time, judging by the clarity of the water and the smell. Invisible problems can occur though and so it helps to do regular water tests using testing kits.

Have I been taking good care of the fish that I have already?

Before adding more fish to your aquarium, ask yourself whether you’ve been looking after your current fish and whether there is anything you could do to improve their conditions. Be honest. There is no point adding more fish to your aquarium if you’ve already begun to lose interest.

If you’ve already suffered from fish loss, then don’t make the mistake of buying more just yet.  If you think that the fish may have died due to a lack of water changes and not enough care, then buying new and more exciting species is unlikely to make you take better care of the tank, and it will be even more disheartening if any new fish die. Take the time to perfect what you have already before getting more adventurous. You can make improvements by properly cleaning your fish tank.

Overfeeding is a big killer of aquarium fish and it’s astonishing just how many people do it unknowingly. As you can probably guess, overfeeding involves putting more food in the tank than your fish will eat. Any left over food will sink to the bottom of the fish tank before decaying and polluting the water.  If you do this often, we’d advise against buying any new fish until the water has cleared and you’ve run a few water tests using appropriate testing kits to ensure the the water is fit for any new fish.

Which species are compatible with one another?

It can be difficult to determine which fish are most compatible with one another because there is so much conflicting information from different sources. Many aquatic stores will sell you any fish that you choose, without warning you of any conflicts that are likely to occur. There are of course obvious rules to follow,  for example most marine fish won’t survive for very long in a tropical fish tank. There are many other fish that are incompatible with one another too. Ghost knife fish for example may look beautiful, but when paired with other fish of different species, they can wreak havoc on the aquarium. Guppies are another species which require a bit of research. Male guppies are prone to chasing female guppies around the tank, which can stress the females and make them ill. Therefore it’s important to ensure that the females outnumber the males, so that all the male attention isn’t focused one the same fish repeatedly. 

How big will the fish grow?

Some fish will look small in the shop, but may grow much larger over time. If upgrading to a bigger aquarium when this happens isn’t an option, then avoid species which can grow greatly in size. Plecos start at 2 inches but can grow huge. Oscars, black ghost knifefish and frontosas are other species which can turn into unexpected giants. Seeing as size can be difficult to determine from photos, it can be interesting to take a trip to a local aquarium to see just how big some of these fish can grow. You may be shocked by their size.

Is the water suitable?

In the days leading up to your visit to the aquatic shop, conduct a few water changes to ensure that there is no ammonia present. Ammonia is an unwelcome chemical which can seriously harm your fish. If after doing a water test, you find that you have ammonia present in the water, we’d recommend waiting another week or so before buying the new fish.

And if you do go ahead and buy new fish, make sure you acclimatise them properly…

Properly introducing fish to an aquarium is an important task. You can’t simply throw them in the water all in one go. Instead you need to acclimatise them gradually so that the change in water conditions doesn’t send them into a state of shock. To do this we recommend floating the bag that they have been transported in on the surface of your tank water for about 20 minutes. Next introduce a few drops of water from your tank into the bag. Repeat this another 20 minutes later, before eventually using a net to move the fish from the bag to the tank.


  • Avatar sharon Posted 03/04/2014 at 2:24 pm | Permalink | Reply

    Please can you tell me how I clean the sand with a lot of plants in tank without disturbing them is it very difficult …thankyou

    • Avatar Cat Byrne Posted 01/07/2014 at 11:54 am | Permalink

      Hi Sharon,

      We recommend a small power gravel cleaner (suitable for the depth of your tank). Gently wafting the nozzle between your plants causing minimum disturbance; unfortunately it is a difficult job and there is no simple solution.

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