Rainwater and Aquariums
Often collected for use in aquariums, rainwater is usually soft and slightly acidic, containing virtually no minerals. Many fish keepers consider rainwater to be a better source of water than tap water, which can contain many undesirable compounds. Unfortunately, rainwater has its own problems and should not be used for aquariums unless it is pre-filtered and supplemented with mineral additives.
Rainwater often contains atmospheric pollutants, has no essential minerals (buffers), and is subject to random events during collection (e.g. dead animals in guttering)