Water Hardness Water hardness refers to the amount of calcium and magnesium salts present in the water. Too much hard water causes a white crust to form on parts of the aquarium. Hard water makes it difficult to adjust the pH. Products are available to help soften the water. Nitrites and Nitrates Nitrites are converted by beneficial bacteria into nitrates. High levels of nitrites indicate that the breakdown of organic materials isn't complete and that the aquarium does not have adequate biological filtration.
Nitrites are less toxic than ammonia, but can still kill fish if the levels are too high.
Nitrates, on the other hand, are generally harmless to freshwater fish and are actually used by plants and algae as food. Test kits are available that test the levels of both nitrites and nitrates. You can buy products that will increase the buffering capacity of your aquarium water. Although salt is associated with marine aquariums, freshwater aquariums can benefit from small doses, as well. One tablespoon per five gallons will provide enough salt for most fish.