Tap water contains chemicals as chlorine or chloramine to make it safe for human beings. These substances are toxic to fish and can weaken or damage fish

Basic water chemistry is pH, hardness and buffering. You should know enough about water chemistry and the specifics of your local water supply so that you can keep fish happy. Every location's water source is different, and some fish won't be able to survive in your water. You can learn details about your water from a local fish store, through the use of test kits, and from local aquarium clubs.

Rapid pH changes stress fish. Tank water has a natural tendency to become acidic due to the production of nitric acid (nitrates) from the nitrogen cycle. Keeping pH stable requires having adequate ``buffering''. If your water is soft, you may need to add buffering agents.

Chemicals that lower pH, e.g. pH-Down, frequently have undesirable side-effects (e.g., stimulate algae growth). If the pH of your tap water is between 6.5 and 7.5, it is just fine for most fish.