Adding Fish to the New Tank

Fish Capacity
There are many different formulas for determining the largest number of fish that should be stocked in a tank. These methods should be used only as guidelines. They are not intended to be viewed as rigid or exact "rules."

When determining the number of fish that should be stocked in a tank, consider the size of tank and its water surface area, the level of filtration and aeration, and the behavioral characteristics of the fish. Experience, and knowledge of individual species is your best guide.

Stocking Order
Stock a new aquarium to only about 25% of the tank’s capacity during the first week or two, then gradually build up to the tank’s full capacity. This will give the tank’s water a chance to mature, and reduce the stress level on fish.

When stocking the new tank, less territorial species should be introduced first, followed by more dominant fish. Hardier fish such as guppies and loaches should also be introduced first.

Make sure that compatible fish are introduced to the same tank. Some fish that are relatively non-aggressive as juveniles will become more territorial as adults, so the "lifetime compatibility" of tankmates should be taken into consideration. (See "Care of Common Fish" at the end of this Aquariums & Fish Section.)

Acclimating Fish To The New Tank
When introducing fish to a new tank, float the plastic bag containing the fish in the tank water for 20 minutes to equalize the water temperatures. During this time, gradually replace some water from the bag with tank water. Submerge the bag completely in the tank, and allow the new fish to swim out. Sprinkle some food in the tank to distract other fish from their new tankmate.

Guidelines For Stocking Fish*

Fish Suggested Tank Stocking Level
Small tropical fish Allow 4- 6 square inches of tank surface (2 inches or less length) (length times width) for every inch of fish.

Medium tropical fish 8-10 square inches of tank surface for (2-5 inches) every inch of fish body length.

Larger tropical fish 10-12 square inches of tank surface for every inch of fish body length.

Cold water fish 8-12 square inches of tank surface for smaller (under five inches body length) fish, and 12-24 square inches for larger fish, depending on size.

Saltwater fish Four gallons of tank capacity for every inch of fish length during first six months after new tank is set up. Subsequent stocking levels can be increased to 2 gallons of tank capacity for every inch of fish length in a mature tank.

* When determining fish length do not include tail and fins. Also, determine length based on the full-grown adult size of fish.