How long to wait until you put a fish in a new tank | Guide

How long should I wait to add fish to the aquarium

Getting a new fish is always exciting. You may want to put your new fish in the tank straight away, but this isn’t a great idea.  Adding a new fish to your tank is a slow process, there are a few different things that need to be taken into consideration first. The newly installed tank should be up and running from a few days up to a week to make sure that all the equipment is working correctly, and the tank is free from leaks. Your tank then needs to be checked first to make sure it is in perfect condition for your fish and the water quality is correct. Once the tank is ready then you need to make sure you know how to acclimatize the fish to the water to not harm the fish.

There are a few important steps that you need to take when adding new fish into a tank, you need to test the water, cycle the water and then help the fish itself to acclimatize. Each one of these steps are just as important as the last so make sure you follow them when adding new fish to your tank.


Fish breath in the same water that they also go the toilet and eat in, this matter all breaks down and produces Ammonia.  Ammonia if left in the water can become toxic to fish and can even kill them if it reaches a certain level. To keep your new fish alive your tank, you need a nitrogen cycle. The cycle takes care of harmful bacteria such as ammonia, nitrite and nitrate. Cycling is the process of the good bacteria colonies within the tank processing the toxic waste and will turn them into non-toxic chemicals. This process can be quite long and could take up to six weeks to fully complete.

The easiest way to understand cycling is that you feed your fish, the fish ingests its food and then produces waste matter. All of the decomposed waste turns into ammonia which can be toxic to the fish. Then the good bacteria will eat the bad chemicals and waste matter, produce nitrates and when you do a partial change of the fish’s water you take out the rest of the bad water. To start cycling you need to have your aquarium set up ready, the good bacteria will already be present within the water but the more bacteria you start with the faster the process. By feeding the good bacteria a good source of ammonia such as fish flakes your colony will start to grow the colony of good bacteria needs to be large enough before adding new fish to fight off the bad toxins within the tank.

By setting up your tank ready, with substrate like gravel and adding plants you will speed up the cycling process. Bactria will also be present in these as well as the water. You can also purchase bacteria in a bottle from most aquarium shops if you would like to speed up the process. The bacteria will have instructions on how to cycle your tank. While your colony of good bacteria is growing you need to be patient and wait before adding new fish into the tank. In the mean time test the waters and monitor the ammonia and nitrate levels and do your research into the perfect fish for your tank and create the perfect tank for your fish. Do not add your new fish into the tank until the tank is ready.

For further information on how to cycle your tank, follow our guide here!

Testing Water

Before adding new fish into a tank, the tank should be installed and you should check to see if the tank is free from leaks, and the filter should be put in so it can mature.  The water in your new tank needs to stabilize before adding new fish. Aquarium water should be treated to help any dissolved gasses to escape and for the pH levels to stabilize. Once you feel your bacteria colony is large enough and has converted the toxic chemicals then you can test the water using a test kit and closely monitoring the peaks of nitrate and ammonia levels.

A testing kit can be purchased at all good pet shops, these include testing strips and an information key to help show you understand the results. Your cycling process will be complete when the chlorine levels are at zero. Also, the ammonia and nitrite concentration levels should stay at 0 and the nitrate should stay below 40 ppm. Se the key inside the testing kit to help you understand the results better. Keep testing the water for a few days and if they continue to stay roughly the same results then you know that the cycle has worked, and you can start adding new fish to your tank.

Once your fish are settles in your tank the good bacteria will continue to grow and deal with, he toxins, you can help by removing and changing 25% of the water each week. This will help to keep the tank and the water clean and stop your fish ingesting harmful toxins.

For more information on water quality and parameters follow our guide here!

Acclimatizing the Fish

Unfortunately, it is very common for fish to die shortly after being added into a new tank. This could be because the fish itself was unwell when purchased, but it could also be because the new owner did not correctly acclimatize their new fish to the tank. Adding new fish to your tank can be tricky, a lot of people place the fish in it’s bag in the new tank, leave it for a few hours and then add it to the water, this isn’t a good way to acclimatize your fish.  

When you bring your new fish home, turn down the lights in the tank to avoid damage or shock to the fish’s eyes. When adding new fish to the tank you should keep the fish in the plastic bag but roll the edges down and place it in the tank water to float. This way it was float steady on top of the water and some of the water that is already in the tank will splash over into the bag. The next thing to consider is the pH levels, even a small difference such as 0.5 in the pH level then this could be fatal to the fish. If the pH levels are close within two tenths then they will equalize very fast, if they aren’t as close it will take longer. When adding new fish to the tank wait, the waters pH levels being equal with give your fish the best chance at survival.

Acclimatizing is a slow process, you should gradually add cups of water from the tank to the bag through the first two hours of the bag floating in the tank. This will help the pH levels to slowly change so it isn’t such a shock to the fish’s body at once. While you are waiting on the pH levels to equalize you can focus on the water temperature. Tropical fish flourish best in waters that are between 76° and 80°F, it’s very important to keep these temperatures so make sure your tank has a precise thermometer. Remember that all species of fish are have slightly different needs so check with the specific species as to what temperature they require before you place them into the tank.

It’s very important to get your water and tank ready before adding new fish. This could be the difference between a healthy, content fish and a sickly fish or even worse, death. The last thing you want is your new pet to suddenly take sick, especially when it’s something that you could prevent yourself. Remember to take the correct steps to cycle and test your water, and to take the time to correctly acclimatize your fish. If you follow these steps, adding new fish to your tank will be the easiest thing in the world.

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