Check your aquarium’s water once a week for its pH, nitrate, nitrite and ammonia levels.
Read up on the chemical tolerances of your particular fish species, so you will know when it’s time for a change of water.
(The people who sold you all the gear may also be able to sell you a water chemical test kit.)
A number of variables affect how often you will need to change the water: how many fish you have, how big they are, the species, the size of the tank, your lighting and the kind of filtration you’re using.
You don’t really change all the water at once.
Just change 10 to 25 per cent of the water in your aquarium, and expect to do it about every two weeks.