Be careful to avoid overcrowding! A good general rule of thumb for stocking the aquarium is: 1 inch for fish per gallon of fresh water. That means that in a bare 10 gallon Aquarium, 10 inches of fish can live comfortably.
Remember decorated aquariums hold less water than the stated amount. With the upgrading of filtration equipment, regular water changes and good knowledge of one's aquarium, this rule of thumb may easily be doubled. Determine the final size of any new fish to ensure they will be suitable for the aquarium. (Marine fish require much more room per fish. Ask anyone on the staff at Bob's for more information.
Be sure the water has been de-chlorinated before fish are added. In new aquariums, add Cycle to immediately commence biological filtration. If chloramine was present in the water, ammonia will be present. Use Amrid or Fluval Ammonia Remover in the filter to help remove it.
In a new set-up, be patient and add a limited number of fish. Start with a few hardy specimens. The biosphere initially is not able to cope with excessive wastes generated by the fish. The system cannot support a full organic load until the natural cycles have had a chance to establish. Regular addition of Cycle bacterial supplement during this initial period hastens the process and reduces the possibility that fish will be poisoned by their decaying wastes. The addition of Cycle helps alleviate one of the greatest difficulties for the new aquarist. After a suitable run-in period (about six weeks), the water will have aged and the biological filter will become established. Some new specimens can be safely added to the community.