How to Acclimate New Aquarium Fish to Your Home Aquarium

It takes time to acclimate new fish to a home aquarium

Person holding a bag of goldfish in their palm

PhotoAlto / James Hardy / Getty Images

It's always exciting to purchase new fish at the pet store to add to your home aquarium. Unfortunately, it's quite common for fish to die shortly after being brought to their new home from the pet store. While many new fish owners are quick to blame the pet store for selling fish in poor health, in reality, this untimely passing is often the result of budding aquarists not knowing how to properly acclimate aquarium fish to their home aquariums.

Most people just float the bag in the tank to equalize the temperature and then dump the fish, water and all, into their tanks. Not only is this a poor way to acclimate fish, but dumping pet store water into your tank is a bad idea. Water from the pet store can contain diseases and parasites that will be transferred to your beautiful community tank at home.

A slow, careful acclimatization process will give your new fish the best chance for survival in your tank, although it will not protect your other fish from any diseases or parasites that the new fish may be carrying. Here's the right way to acclimate fish to their new home.


All new fish being put into a community tank should be quarantined for at least two weeks before adding them into your aquarium. If you don't have extra tanks to quarantine them in, you must be very observant of the condition of the fish dealer's tanks. If there are sick fish, dead fish, or ich on any of the fish in the dealer's tanks, do not buy fish there and put them in your community tank.

How to Add New Fish to an Aquarium

  1. Open the Bag

    When you get your new fish home, turn down the lights to avoid shocking your new pet. Your next step should be to take off the rubber band and open the bag. Place the bag in the tank, so the water supports it. Next, roll the open top of the bag down four or five turns, so it creates a ring of air trapped in the rolls of the plastic bag. Now the bag will float on its own without tipping over. If it is still unstable, a couple more rolls might be needed.

  2. Add Water From the Tank to the New Bag

    Acclimating is a slow, steady process. To begin, dip 1/2 cup of tank water from the tank and add it to the bag. Now wait 15 minutes and do it again. This slow process will allow your new pet to acclimatize to a change in pH and temperature as well as new nutrient levels, oxygen content, salinity, sounds, and lighting.

  3. Check Water Quality

    Check the water quality in your aquarium to ensure it is appropriate for the fish. You should test the ammonia, nitrite, nitrate, hardness, alkalinity, pH, and temperature on a regular basis. You should only think about adding new fish to your aquarium when all of the water quality parameters are at the correct levels.


    Adding new fish into the aquarium will increase the bioload on the filter system, so only a few new fish should be added to the aquarium at a time. This allows the beneficial bacteria in the biofilter to grow enough to remove the additional ammonia waste the new fish produce before it can build up to toxic levels. Adding many fish at once can overload the biofilter and cause fish loss from ammonia increases.

  4. Test the pH

    To avoid pH (acid/base balance) shock, while the temperature is equalizing, you need to test the pH of the water in the bag and compare it to the pH of your home tank water. In most cases, for a short trip of an hour or less from the pet store to your home, there will be little change in the water quality in the transport bag. For long trips, such as when the fish are shipped from the breeding farm through the distributor then to the pet store, the fish may be in the bag for over 24 hours and up to a few days. In these cases, the pH of the water will drop because of the fish's respiration, and the ammonia level in the water will increase.

  5. Address pH Imbalances As Needed

    With a difference in the pH of less than 0.4 units, you can add the fish after doing the initial water mixing and the temperatures are equilibrated. With a larger difference in pH, like 1.0 unit or larger, you will need to add more water into the bag until the pH is close to that in the aquarium. The measure of pH is exponential, so a difference of one unit on the pH scale is ten times the level of acidity between the two water samples being tested. Discus fish are a prime example of a fish species that need a long acclimatizing period because they require a lower pH (more acidic) water than most other fish species.

  6. Add the Fish to the Aquarium

    Once the temperature and pH of the water in the bag are similar to the water in the aquarium, remove the fish bag from the tank and use a net to take the fish out of the bag. Add the fish into the aquarium carefully, ensuring that their fins do not get caught in the net mesh.


    Do not pour the water from the bag into the aquarium; simply discard it. If removing the bag and its water decreases the level of water in your aquarium, add fresh dechlorinated water to top of the tank level.

  7. Monitor the Fish for Bullying

    It is important to observe your new fish to be sure they are not being picked on by other fish in the aquarium. It is also helpful to feed your fish a small amount of food at this time so that the current fish are busy eating and less likely to bother the new fish while they settle in. Remember, it is always better to place new fish into a quarantine tank for 2-4 weeks before adding them to the main aquarium with your other fish. Use this same technique for introducing the new fish into your filtered quarantine tank.

A woman testing pH levels in aquarium
Elva Etienne / Getty Images

Preventing Problems With Your Fish During Acclimation

More new fish have been killed by pH shock than from any other problem after adding them to an aquarium. Differences in pH, while they may not sound significant, can be lethal to a fish. In fact, as little as a 1.0 difference can send your fish into pH shock. This is something they may recover from, or may not, depending on the severity of the difference in pHs. The bigger the difference, the more chance of your new fish dying from it.

Try to learn what the proper pH is for your fish species (most are in the 7.0 to 8.0 range, but some like higher levels and some lower). Also, over time, the pH in the water will naturally drop due to acids produced by fish metabolism. Regularly performed water changes (25% or more per month) will keep the pH more stable and replenish the alkalinity (pH buffering) in the water. If your local water supply is soft (low alkalinity and hardness) you may need to add an alkalinity buffer into the aquarium water periodically to maintain the proper pH for your fish.


During transportation, the water in the fish bag will drop in pH and increase in ammonia. Making the adjustment to different water qualities as easy for your fish as possible will help reduce stress and losses. You could even drop a few crystals of ammo-lock (or similar ammonia neutralizing product) into the transport bag to detoxify any excess ammonia if it was a long trip home from the fish store.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  1. Aquarium Water Quality: pH. Florida Department of Agriculture and Consumer Services.