If you are going to keep healthy fish, there are a few things you need to know about the water they'll be swimming in. Aquarium fish are in a little glass box; that is not nature. Because it is not natural, you need to be extra careful about testing the water and keeping it clean. You need to be careful not to feed more than they can eat while you watch, and you need to remove dead plants and fish as soon as possible. You need to vacuum the gravel once a month (daily, for a fishbowl). You need to replace some old water with fresh each week; 20% is good. You need to do chemical tests with test kits to be sure there is no ammonia or nitrite building up in the water. (See "The Nitrogen Cycle," below.
Most tapwater has been treated with Chlorine to kill harmful bacteria that can make you sick. That is good for you, but not for your aquarium. Chlorine kills fish and the helpful bacteria that keep the water clear of toxins (nitrobacters). You must remove the Chlorine before you put it into your tank. Dechlorinators are sold for this purpose. Some are better than others. Some smell bad, some interact with your test kits to give false results, and some are irritating to your fish. Some of the declorinators that say they stimulate the skin's protective slime coat are actually irritating the fish's skin so the fish produces extra slime coat to protect itself! The best declorinator I have found is Stress Coat made by Aquarium Pharmeceuticals. It does none of the above, and has Aloe Vera in it which actually makes my hands feel better after I've had my hands in the bucket of treated water. (Do as I say, and not as I do - you shouldn't have your hands in the water; you may have dirt, soap, oils or other stuff on your hands and you don't want to get those into the water).
You need to know the pH of the tank water: that is a measure of how acid or alkaline the water is. Pure distilled water has a pH of 7.0. Acid loving fish, like Angelfish or Discus will want water closer to 6.0 and alkaline loving fish like Cichlids from Lake Malawi need water closer to 8.0 or higher. That is a big difference: 7.0 is ten times more alkaline than 6.0, and ten times less than 8.0, so 6.0 is one hundred times more acid than 8.0. The difference comes from where the fish are native to: Angelfish and Discus come from the Amazon in South America, and Lake Malawi is an alkaline lake in Africa. Because of the difference in pH needs, you can't put a Discus in with a Lake Malawi Cichlid! You have to know all about the fish BEFORE you buy anything. Most other common community fish have been bread in captivity long enough that they are not so picky about their native pH. They do care, however, if the pH you have changes. Fish like stability. They hate when things change.
You need to test for hardness, too. GH is hardness (the dissolved mineral content) and KH is the carbonate hardness (the amount of carbonates dissolved in the water). The same rules for pH apply to hardness. Know what your fish need, and make sure your levels stay stable and don't change much. It is a good idea to match the fishes' needs to the qualities of the water that comes out of your tap, so you don't have to work too hard to adjust it.
THE MOST IMPORTANT TESTS YOU NEED TO DO ARE AMMONIA AND NITRITE. These two chemicals are a product of normal fish biology (breathing, urinating, etc.) and the decomposition of food and plant materials. Because of this, it is always being made in your aquarium, but it is DEADLY to your fish! ...In come the Nitrobacters, heroes to all fish in glass boxes everywhere...
What is a nitrobacter, you say? They are a bacteria, and you will grow to love them. Your aquarium needs a whole colony of nitrobacters, because one kind of nitrobacter eats ammonia and another kind of nitrobacter eats nitrite, so with enough nitrobacters populating your filter media, both of these poisons will be consumed as soon as your fish make them, so the water will stay clean and healthy. Ammonia and nitrite MUST BOTH ALWAYS TEST ZERO. If you have them, you don't have a big enough nitrobacter colony, so you will need to change water more frequently and use ammonia removing chips in your filter until the colony can grow. If you have an overcrowded, poorly filtered, dirty aquarium, the nitrobacters won't keep up at all. The stress from the poisons will weaken the fish, and they will start coming down with all sorts of diseases.
To help your nitrobacter colony grow, they will need somewhere to live. That is what your filter is for. The foam block and activated charcoal or other media in your filter has a lot of surface area on which the bacteria can grow. That is what your "biofilter" is. A biofilter is nothing more than the media in your filter after the colony of nitrobacters have moved in. As the water filters through, the ammonia and nitrite are eaten by the nitrobacters and the clean water goes back into your tank. Nitrobacters. You need them. You love them. The result of all this ammonia eating is a chemical called nitrate. (NitrAte is completely different from nitrIte. ) Nitrate is also poisonous to your fish, but much less so, so you have time to take it out with regular water changes, and it does not have to test zero. All of this nonsense I just taught you is called "The Nitrogen Cycle."
To read this diagram, start at the top left corner and work your way clockwise. The fish make wastes (ammonia) that the first kind of nitrobacter eats. That nitrobacter makes a waste (nitrite) that the second kind of nitrobacter eats. The second nitrobacter makes waste (nitrate) that you have to take out with regular water changes. Some leftover nitrate may feed your plants (or algae!) and if the fish eat that, they will make more waste (ammonia) and the wheel keeps going around.
YOU CAN NOT PUT YOUR FISH INTO YOUR AQUARIUM UNTIL YOUR TANK IS "CYCLED" ! That means you can't put fish in if you don't have a big enough nitrobacter colony to clean the water for them, so you need to grow the bacteria (cycle your tank) first. There are a few ways to do this. You can learn more in the FAQ section.
FILTERS AND STUFF
THE NITROGEN CYCLE
DISEASES AND MEDICATIONS
PLANTS AND ALGAE