To Acclimate or Not to Acclimate

By: Mike Sergey

Acclimation of fish is a topic of much debate. Some people acclimate new arrivals for hours, then give them a freshwater dip and keep them in a quarantine tank for weeks. Some people just open the bags and dump both water and fish in the tank. In both cases, sometimes the fish are fine and sometimes they are not.
So What Is the Correct Way to Acclimate New Fish?
As with most topics in this hobby, there is no answer that fits all fish. Depending on the type of fish and how long the fish has been in transit, the acclimation technique may change slightly.

First, make sure the fish are healthy and were not caught with harmful drugs. You will have a much easier time acclimating fish that were conditioned before they were packed for you.

Next, acclimation should take place under a very dim light. The fish should remain under dim light until they exhibit normal swimming behavior, both in the acclimation container and after they are introduced into the aquarium.

For most species, the bags should be floated for 10 to 15 minutes in the tank. That will compensate for any water temperature difference.

Do not acclimate too long!
I recommend that acclimation be kept to a maximum of 20 minutes. Acclimation in excess of 20 minutes can be harmful due to depletion of dissolved oxygen in the acclimation container. Do not attempt to boost dissolved oxygen by aerating spent shipment water - it already contains relatively high amounts of fish waste (ammonia), and aeration can cause a rapid increase in pH, resulting in increased ammonia toxicity and stress for the animals.

Open the bags, add 20% tank water to the bag and wait five minutes. The animals should simply be netted directly into the dark or dimly lit tank. Severely stressed fish, usually indicated by rapid gill movement or shallow breathing, quivering of the pectoral fins and/or the inability to remain upright (lack of equilibrium), are best kept near the surface of the water, where dissolved oxygen is the highest. This can be done by suspending the fish in a perforated floating plastic tray.

New arrivals should not be exposed to bright lights, strong aeration, stressful chemical agents or established, territorial livestock.

Do not over -acclimate your fish. Ammonia toxicity is very stressful and will kill fish. Watch to make sure the new fish is not attacked by other established tank mates. New fish should be added to the bottom of the aquarium, not at the top. Your fish are used to you adding food at the top of the aquarium may attack out of instinct. By adding your fish with a net to the bottom, you accomplish two things: first, your other fish get scared, and second, your new fish can find a hiding place until he is ready to venture out.

This is the best general acclimation technique I have found. Remember, certain fish may require different procedures, but in most cases, shorter is better.

Start with good healthy fish, acclimate them into a healthy aquarium, and both you and your fish will be happy!