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Not changing the water regularly is the most common cause of illness in fish, so this is one of the most important things you can do to keep your fish healthy. Monitor the water quality and ammonia, nitrite and nitrate levels by using water testing kits from your store. This will tell you how often and how much you need to change. Never change all of your tank water at one time. This change in water chemistry will likely stress your fish. You should not change more than 1/3 of the water in any 24 hour period. Some aquariums can do ok with changing 1/4 of the water every two weeks. However, most pet fish owners will need to do it more often. Changing 25% of your tank's water every other week helps to dilute and remove nitrates as well as replace trace elements and important buffers used up by bacteria. You will also need to remove wastes hiding in the nooks and crannies of your aquarium. To do this, vacuum the gravel when you change the water. The exception to this is saltwater aquariums that use live substrates at the bottom. If your filter is not removing ammonia properly because it is clogged, your fish will get stressed and eventually die. Cleaning the filter usually involves either rinsing them in used tank water or vacuuming them. Tap water has chlorine or chloramine in it so it's safe for us to drink. However, these chemicals are toxic to fish and damages their gills, causing stress and illness. You will need to add the chemical sodium thiosulfate, available at fish stores, to the water before adding tap water to your tank. This will neutralize chlorine in the tap water. Chloramine can be neutralized through the use of products such as Amquel. These chemicals take care of the ammonia and chlorine in the chloramine molecules. If you don't want to use chemicals to remove chlorine, you can leave circulating water via a filter or air-stone in a bucket or tank for 24 hours. Your fish will get stressed from rapid pH changes in tank water. Aim for a pH between 6.5 and 7.5. This is ideal for most fish. Tank water tends to become acidic over time because of nitrates that build up. The pH can be raised or lowered using chemicals such as muriatic (hydrochloric) acid or phosphoric acid. Phosphoric acid can raise the phosphate level in your tank and cause algae growth. Always treat water for pH adjustments prior to adding it to the tank. You can also bubble CO2 (carbon dioxide) through the tank via CO2 injection systems. This will lower the pH without the need for chemicals. Aquatic plants can help your aquarium establish its own natural eco-system, protect from early fish loss, release oxygen, keep algae at bay and purify your water. Plants can also make your tank look really cool! An aeration unit may not be required if you have healthy, aquatic plants in your tank. Aquatic plants will use up toxic ammonia and nitrites in your tank to grow. Faster growing plants like Cabomba, Ludwigia or Egeria Densa or other stem species can remove a lot of ammonia quickly. Your fish will benefit from having algae eaters control potentially harmful algae in the tank. These include shrimps, snails, and algae-eating fish.
Change part of the water regularly. Perform regular maintenance on your filter. Treat tap water. Keep the water pH stable. Add plants. Add algae eaters.