Depending on a variety of factors, including the number of animals kept, the size of your aquarium and the type of filtration used, filter pads and other filter materials may need to be cleaned or replaced on a monthly basis. An easy way to check if the activated carbon in the filter is still effective is to release a few drops of methylene blue (available as a fish medication at pet stores) near the filter's intake. If the water leaving the filter shortly thereafter is blue, you will know that the activated carbon is exhausted and needs to be replaced. Clear water exiting your filter will indicate that the carbon is still effectively removing pollutants from the water. You should also perform a partial water change on a monthly basis, or more frequently.
Many fish can tolerate nitrate levels of at least 30 mg/liter, but, ideally, the levels in your aquarium should be maintained below 5 mg/l and certainly no higher than 10 mg/l. This is especially important with invertebrates. While sublethal levels of nitrate may not kill your pets, they can subject them to stress and may inhibit growth, breeding, and overall health.
The condition of aquarium decorations such as plastic plants and coral skeletons should be observed at this time also. Any materials that are porous or that contain numerous crevices, such as coral skeletons, may need to be rinsed or more thoroughly cleaned. If the organic debris that has accumulated within a coral skeleton cannot be removed by simple rinsing, the material may be immersed in a solution of 8% bleach and water overnight and then carefully rinsed with fresh water before being returned to the aquarium.
Nitrate, ammonia, and nitrite levels should also be taken on a monthly basis. In the early stages of your aquarium's establishment, these tests should be made more frequently, as outlined in Chapter 2. As you become more familiar with your aquarium and its specific water chemistry, you may be able to monitor it adequately by testing nitrate levels on a monthly basis and ammonia and nitrite levels on a bimonthly basis.