There are no hard and fast rules about how often to clean your aquarium. It depends on several factors, including size, filter system and number and type of fish. There are some general guidelines to follow, however, that will help you create a schedule that works for your aquarium.
Daily
It's not necessary to clean daily, but making daily observations can help you determine whether or not the tank is being cleaned often enough to keep your fish healthy. Observe the fish for a few moments and make sure they all look healthy and are behaving as usual, with good color, clean scales and fins that are not clamped, slimy or ragged. Remove any dead fish right away. If your fish look stressed, sluggish or are gulping for air, it's time to perform a partial water change, even if it's been less than a week since the last cleaning.
Weekly
In large, well-established aquariums it may be possible to change the water every other week, or even as little as every month, but in most cases a water change should be done once per week to keep the tank clean. Remove about one-fourth of the water and replace it with treated water. Use an aquarium vacuum to clean the gravel and a sponge or scraper to remove excess algae from decorations.
Monthly
If your aquarium is established and the fish are healthy, water only needs to be tested once per month for nitrate, ammonia and pH levels. Keep a record and make sure they remain fairly steady from month to month. Test more frequently if the levels start to shift, or if your fish look stressed or unhealthy, or the water suddenly becomes cloudy or colored. Clean filter cartridges or media once each month, using water removed during the water change. Check air stones for build-up and either clean them by boiling in clean water or replace them.
Semiannually
Twice per year, turn off and unplug all of the equipment and give it a thorough inspection, cleaning as necessary. This includes hoods and light fixtures, pumps and filters. Replace light bulbs and filter media as necessary and make sure everything is in good working order. Wipe down the hood and the pump housing to remove dust, dirt and water spots.
Deep Cleaning
Deep cleaning, in which everything is removed from the tank and scrubbed clean, should be avoided except under special circumstances. Such a cleaning is not only stressful to the fish, but it kills off the beneficial bacteria that consume fish waste and keep nitrate levels low. Regular cleaning, which includes partial water changes and cleaning filter media about once per week, should eliminate the need for deep cleaning. The only time a deep cleaning is recommended is in case of an outbreak of disease that cannot be controlled by other means.