Prepare the tank by testing the pH, ammonia and chlorine levels. Chlorine and ammonia must be zero. Also, verify that the water temperature is suitable for the new fish. You will use the pH results when acclimating the fish. If there is ammonia or chlorine in the aquarium use a dechlorinator and ammonia binder treatments.
Turn off the light in the aquarium to reduce stress on the new fish. Also, dim the lights in the room, if possible, or direct light away from the tank.
Place the sealed bag containing the fish into the tank water so the bag floats. Let the bag float for 15 minutes to allow the water temperature in the bag to equalize to the aquarium temperature.
Open the top of the bag without letting any water into or out of the bag. Fold the open end of the bag over to create a hem with an air pocket to allow the bag to float upright. If necessary, fold the hem again.
Test the pH of the water in the bag and in the aquarium. Compare the result to the pH of the tank water, and note the difference. For example, if the bag water pH is 8.0, and the tank pH is 8.2, the difference is 0.2.
Drip a 1/2-cup measuring cup of tank water and pour into the open bag. Wait 15 minutes. Repeat the same process as many times as needed to balance the pH of the bag water, based on the initial pH difference.
When adding the new fish into the tank it is recommended to change around rock to allow defensive fish to let the newbies settle in nicely.
Use a small net to lift the fish out of the bag and quickly transfer them to the aquarium. Brine shrimp nets work well, but if you have only a large net, hold the net over a bucket and gently pour the fish and water out of the bag and into the net. Then quickly transfer the fish from the net to the tank.
Discard the bag of water in the sink. Do not pour the water into the aquarium.
Leave the light off for several hours to allow the fish to adjust to the new setting.
**Adding a small amount of food into the aquarium to distract the current fish from the new fish is also helpful.