Cleaning your tank
When cleaning your aquarium, you should just remove 10-15%of the water and replace it with fresh, dechlorinated tap water (bowls require larger water changes more often). While you are doing this, you should use your siphon to suck up some of the gunk that collects in the gravel and decorations. If you have an under gravel filter, it is very important to clean the gravel when you do your weekly water changes, this will prevent detritus and other decaying organic matter from blocking the passages between the pebbles and restricting water flow. Generally, you can clean 25% of the gravel while siphoning out 10-15% of the water.
If you have algae growing on the surface of the tank or ornaments, you should get an algae scraper or magnet cleaner of some sort and scrub the glass before removing water. Many varieties of algae scrapers or scrubbers are available at your local Aquatics store. For additional cleanliness, you could get algae eating catfish and/or scavengers to pick up some of this work for you, however, having catfish in the tank does NOT mean that you don't need to clean. In fact, algae eaters and catfish, like any other fish, will add to the biological load of your tank and increase maintenance requirements.
If you have to clean out the filter(s) do not change all the media (cartridges, sponges, carbon packets, etc.) at once, as this will remove the majority of your biological filter, causing the tank to have to go through a cycling period again to get bacteria reestablished. Rinse any new filter media in cool running water before introducing it to the system (unless the instructions for the filter media specifically state not to).
Once you have siphoned 10-15% of the water from your tank, you will need to fill it again. The easiest and neatest way to do this, is to use a siphon to siphon water from a bucket into the tank. This will tend to reduce spilling and messing up the gravel and decorations. Use a bucket that has never had detergents or household chemicals in it and fill it with water. The water should be close to the temperature of the tank water that you just took out. Use a chlorine remover to prepare the water for introduction into the tank. Place the bucket somewhere higher than the top of the fish tank, and get your siphon going again and in just a couple of minutes, you should have a full tank. Be sure to watch the siphon, in case the hose gets bumped out of the tank, or if there is enough water in your bucket to overfill the tank. Remember, there needs to be some space between the top of the water and the aquarium cover, because your fish rely on oxygen exchange at the surface of the water in order to be able to breathe.
Do not just "top up the tank" to replace water that "disappears." The water that is disappearing is evaporating, and it leaves behind all the impurities that were in it to begin with. This means that as you just top up the tank, you are making your water harder until it will eventually no longer be able to support fish. Additionally, if there are even trace amounts of heavy metals or other toxic substances in the water, you are giving your fish more and more of these every time you top off the tank. Most municipal water systems have at trace levels of at least one potentially hazardous substance, but in minuscule amounts these should never be trouble for you or your fish. Furthermore, by not removing water from the tank from time to time, you allow build up of waste products not removed by the filter (such as Nitrate), which are potentially hazardous to the fish and encourage algae growth.
Remember, cleaning your tank is easy. Scrub for algae, remove 10-15% of the water while cleaning the gravel and top up the tank with dechlorinated water.
These cleaning tips are intended for tanks that are already cycled and are over about 45 Litres or 10 gallons in size. Care requirements may vary for tanks that are still cycling or for small aquariums and fish bowls.
