Fish don't like swimming in their own waste any more than people would. Nature breaks down fish waste in the wild. Put the fish in a tank and filtration of the tank becomes a human responsibility. Natural plants help filtration, but some fish require artificial plants. Luckily, there are three main kinds of filters that provide sufficient chemical, biological and mechanical filtration in most aquariums: power filters, under-gravel filters and canister filters.
Rinse the filter (even if new) in a bucket. Combine tank water with vinegar (a natural cleanser that does no harm to fish). Tank water should already be free of the metals contained in tap water, so filters rinsed this way will also be safe.
Rinse the filter media in the same vinegar and water solution for 15 minutes, even if only one filter cartridge is needed.
Assemble filter media. All necessary parts---foam and carbon usually---will be in the filter box when purchased. Some power filters have one insert that contains both media. Others require assembly of the media for insertion to the filter chamber.
Gently slide media in to the empty filter chamber.
Fill the chamber with tank water.
Plug filter into the power outlet. Ensure that air bubbles disappear and filtered water begins to flow from the filter into the tank. This can take one or two minutes.
Rinse the filter---even if new---in a bucket. Combine tank water with vinegar.
Rinse substrate (gravel) in a bucket of tank water. Swirl the gravel around the bucket using your hand like a claw. Ensure that all gravel is rinsed by swirl for several minutes.
Assemble filter if needed. Often, under-gravel filters will come in two pieces that snap together. Ensure that tubing insert fittings are on the outside corners of the filter, which will occupy the rear corners of the aquarium.
Place filter on the bottom of the bare aquarium.
Connect air tube pipes to the intake and outflow (corner) sockets of the filter. Intake tubing is smaller than the outflow fittings.
Pour substrate (gravel) into aquarium up to a depth of two inches. Distribute gravel evenly along the tank bottom.
Attach opposite ends of aquarium tubing to either side of the T-valve joint. Fasten a short piece of tubing to the remaining T-valve fitting. This merges two air passages (coming from the tank) into the one that connects to the air pump.
Fill the tank with water to at least a level higher than the filter's outflow pipes.
Plug in the air pump. Confirm that bubbles are emitted from outflow pipes to ensure proper operation.
Rinse the filter media in a bucket containing vinegar and tank water.
Insert chemical and biological filtration to the canister unit. Check the manufacturer's instructions on which media goes where in the assembly, as different designs have different locations. Think of it as running dirty water through the elements so you can clean it for use in the tank.
Assemble intake and outflow piping.
Slide suction cups on the tubing unit, as they will hold the pipes along the glass sides of the tank.
Place tubing assembly into the tank, ensuring that the "spray bar" openings are facing toward the front of the tank. This is where the air shoots out. Keep the "check valve" above the tank's water line, so you can adjust air flow accordingly.
Slide air hose to the tubing assembly. Leave enough room to use clamps (provided) to secure the hose-to-tube connection.
Fasten the hose to the tubing assemblies using the clamps.
Attach the hoses to the canister itself. The canister's intake valve is grey and the output valve is black.
Fasten the hoses to the canister valves using the remaining clamps.
"Prime" the filter by filling tubes with water using a funnel (most filter kits include one).
Plug in the canister filter. Inspect intake and output hoses to ensure that air bubbles are pumped out. This may take a minute or two.
Ensure that gravel pebbles are larger than the holes in the grid of the under-gravel filter. If pebbles slip through the grid, the filter won't work as well. Change filter media monthly. Changes may need to be more frequent depending on which type of filter you use, the size of your tank, and how many fish are producing waste in it. Check with your local retailer. Canister filter: use pliers to secure hoses to filter tubing if necessary. Hoses can be stiff at first, but they provide a tight seal.
Rinse all substrate at least once before using it in the aquarium. Gravel sold at most aquarium retailers is pre-washed, but you need to do at least one more rinse. Unwashed gravel may need up to 20 separate rinses before it is safe for aquarium use. Save yourself the headache and buy pre-washed gravel. Establish healthy bacteria in the tank before adding any fish to a new aquarium. Consult your local aquarium retailer on how to do this to avoid "new aquarium syndrome," which can be fatal to fish.