It is a good idea to leave a newly installed aquarium for a week to ensure that all the electrical equipment is working properly, the tank is free of any leaks and the water is free of chlorine and the supersaturated nitrogen than comes out of solution as bubbles in a newly filled aquarium. Although nothing looks quite as empty as a new aquarium, this time can be put to good use deciding which fish to add and in which order, as after all, it's much easier to put the right fish in than take the wrong ones out!
As a new tank is incapable of supporting fish until the filter matures, at this point you have a couple of options. A great method, if you have the patience, is to start the system maturing before adding any fish by using an ammonia source and a bacterial culture. This technique requires a test kit and close monitoring of peaks in both ammonia and nitrite levels before adding your first fish but has the advantage of taking your tank through the dreaded 'New Tank Syndrome' before you have any fish in there. Although this avoids exposing your new pets to damaging water quality problems, it can be a very frustrating process, sometimes taking weeks to complete. A more efficient way of accomplishing the same end result is to add mature filter media to the system and this is often available from our stores in the form of large chunks of ceramic media that can be placed in the aquarium and left to colonise the filter. A few manufacturers now supply bacterial cultures which can be added as the first fish are placed in the tank as a further option and this can make all the difference in the early days of your aquarium when you are most likely to encounter water quality issues. Note that whichever method you choose, bacteria need a source of fish waste to thrive and will die without either the addition of a small quantity of live fish, or further additions of ammonia.
The most established method of maturing a new aquarium is by the addition of a small number of hardy fish. The resulting ammonia and nitrite spikes are then observed and further fishes are added when these pollutants return to zero, with further gradual introductions taking place when tests prove negative for ammonia and nitrite. This tried and tested technique exposes fish to level of pollution that can prove stressful and is starting to lose favour now that better options are available. Whichever technique you choose, using a test kit is a vital part of setting up a successful aquarium.