Aquarium should be filled with clear potable water, quality depending on the types of fishes being kept there. Tap water is probably the safest source as aquarium water for the majority of the tropical fish and plants. But it contains chlorine, which even in quite low concentrations can be fatal to the fish. To remove the chlorine naturally, it is better to allow maturing the water for few days (aging) or aerating overnight before addition. During emergency dechlorination can be done with the commercially available chemical purchases from the pet shop with satisfactory results.

Direct pouring of water stirs the sand-bed and gravel making the water turbid. To avoid this, a shallow pot may be kept on the sand and water be gently poured into this. It will overflow gently over the brim and fill the tank without disturbing the sand.

The most satisfactory method is probably to ensure that the aquarium is set up and left for 24 hours, with the aeration or filtration working. It will also help in checking the working of the equipment like filter, aerator, and toys etc. before introducing fish.