Filter maintenance
The filter in your aquarium will require regular maintenance to keep it running effectively and to prevent any breakdowns. A well maintained filter will last years longer than a poorly maintained one. There are two main parts of the filter that need to be regularly checked; the media and the impellor.

How often does filter media need maintenance?
The sponge(s) within the filter will need to be periodically cleaned to prevent them from clogging and reducing the water flow. A slightly dirty sponge however, is more effective than a brand new, or clean sponge. The bacteria that convert toxic pollutants into less harmful chemicals will thrive in dirty conditions. However, if the sponge becomes to dirty, the flow rate will slow down and other, less useful bacteria will grow. This results in a drop in oxygen levels and the useful bacteria may suffer.
Internal filters, which generally have smaller sponges and less powerful pumps, will need to be cleaned at least fortnightly. External filters however, have larger sponges and more powerful pumps, so are less likely to clog as quickly. External filters under normal aquarium conditions need only be cleaned every 3-4 weeks.
There are of course, other types of filter media than sponges, particularly for external filters. Carbon or carbon sponges (Normally black sponges) should be replaced at least every 2-3 months, after this time, they may begin to release chemicals that have previously been removed by the media. Filter-floss medias (cotton-wool like sponges) should be replaced every week if possible, but this does depend on the individual aquarium conditions. Biological media, which is normally in the form of ceramic or sintered glass (which confusingly, does not look at all like glass) nodules, should be cleaned every month. Biological media should only be lightly rinsed however, not thoroughly cleaned.

What's the best method of cleaning media?
It is very important that filter media is only cleaned in water taken directly from the aquarium. Tapwater contains chlorine, which is added to kill harmful bacteria, which could be dangerous to humans. Chlorine is indiscriminate however, and also kills the useful bacteria found within the filter media. Dechlorinated tapwater can be used but there are other variables that may be harmful to the bacteria such as changes in temperature, pH and hardness of the water. The best method by far, is to remove some water from the aquarium into a container (a washing-up bowl or bucket will do) and rinse the sponges in that water. Ideally, the container should be bought new and used only for aquarium use; this will prevent the risk of introducing harmful chemicals such as detergents to the aquarium. As mentioned before, the sponges need not be thoroughly cleaned, but just quickly squeezed a few times to remove the majority of debris.

Maintaining the impellor
Almost all standard aquarium water pumps, including those that power filters contain an impellor. The impeller has a cylindrical magnet attached to a series of blades, which are used to move the water. Most filters have a removable cover over the impellor, which is situated in the top of the filter. The impellor is normally fixed onto a metal or ceramic shaft and can be simply slid out for cleaning. The removal of the impellor should be done with a little care as ceramic shafts can sometimes snap.
Over time, algae, bacteria, and other debris can collect around the impellor, which will impede the performance of the pump. Cleaning the impellor is relatively simple; a gentle rub with fingers or a cloth will remove most build-up. A pipe cleaner or cotton bud can be used to clean the impellor housing and around the impellor shaft.
Thankfully, the impellor need only be cleaned every two or three months. It is a small job however, and could be done whenever the media is cleaned. A dirty impellor will place a strain on the pump and considerably reduce its life span.

Cleaning the substrate
A good filter will remove a substantial amount of waste matter but in most cases, the majority will end up in the substrate. Waste from fish and from food collects in the substrate and builds up, eventually decomposing and releasing pollutants. These pollutants will encourage algae and can cause bacterial infections in bottom dwelling catfish and loaches. To keep the amount of mulm or debris in the substrate to a minimal amount, a special device called a gravel cleaner can be used. Gravel cleaners may vary in size, shape and functionality but they all work on the same basic principle. A gravel cleaner uses the force of suction to siphon water from the aquarium. When the gravel cleaner is placed in the gravel, it pulls up any mulm and debris along with the water. The gravel cleaner can then be lifted, allowing the heavier gravel to fall back down, and moved on to another area. Because a significant amount of water is removed from the aquarium whilst using a gravel cleaner, it is wise to use gravel cleaning as part of a water change. It should be possible to clean the entire substrate whilst removing only about 10-15% of the aquarium water. Ideally, this should be done once a week to keep the gravel in top condition and prevent algal growth. For aquariums with heavy planting, gravel cleaning can be disruptive and damaging to the plants. In these cases, only the very top layer of gravel in the area of planting should be cleaned. Although debris will build up beneath the plants roots, the waste will provide the plants with useful nutrients.

Water changes
Despite stories you may hear from other fishkeepers who have kept aquariums for months on end without water changes, regular water changes are vital to the health of your aquarium. With the exception of a few circumstances, best left to well experienced fishkeepers, water changes should be carried out at regular intervals. The amount of water you should change and the frequency of changes depends on individual aquarium conditions. This is one reason why there are many different figures given as advice. As a general amount, around 10-20% every week is normally suitable for most tropical freshwater aquariums. Heavily planted aquariums may need only a 10-15% change once a fortnight.
The function of water changes is to remove contaminants such as nitrates, which build up over time in the aquarium, and to replace lost 'trace elements' and minerals. Without water changes, your aquarium may appear fine for a number of months but this is only because the detrimental effects are accumulative. Throughout long periods without water changes, nitrates will steadily rise whilst minerals are used up, causing the water to slowly become softer and more acidic. Eventually, the aquarium water may be unable to 'buffer' any pH changes, causing dangerous pH fluctuations and high nitrates will begin to cause disease problems. The effect of few or no water changes is often seen when fishkeepers introduce new fish into their seemingly healthy aquarium. Because the fish already in the aquarium have slowly become 'used' to the rising nitrates, they are not visibly affected. The new fish however, are not used to the high nitrates and it will be quite a shock to them. A few days after being introduced, the new fish may die or succumb to disease. This often has the effect of causing existing fish to become infected by disease, because their immune system is weakened by high nitrates, and also begin to quickly die. To the fishkeeper, the aquarium was fine until the new fish were introduced, and the retailer often receives the blame for the loss of fish. The cause in fact, is the lack of water changes over the previous months.

Preparing water
To carry out a water change, the new water must be suitable for use in the aquarium. Raw, untreated tapwater is highly unsuitable as it is much cooler than the aquarium water and contains dangerous chlorine, chloramines and metals. To prepare tapwater, a dechlorinator must be added. Dechlorinators are simple liquid additives, available from all fish retailers. Most good dechlorinators will remove chloramines and metals as well as chlorine. Although dechlorinators work immediately, the tapwater is still too cold to be added straight into the aquarium. Sudden temperature changes can cause stress and disease in less hardy fish so should be avoided when possible. To warm the water, it should be left overnight in a warm room. Although it will still be cooler than the aquarium water, the difference is far less than before and small enough to avoid damage to the fishes. The container used to hold water should be bought new and used only for the aquarium to avoid the introduction of any dangerous chemicals

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