Cleaning aquarium filters correctly ensures the established beneficial bacteria are not disrupted too much causing an ammonia spike in your aquarium.
Check the water flows of power filters during regular maintenance for evidence of slowing.
If your filter is an integral type located under the top cover, this type of filter’s water flow isn’t affected by clogging. You will find that the water will flow across the top of the filter media between the sections instead of taking the correct path of under-over-under or something similar.
Internal air lift type filters are harder to check for water flow because as they get clogged, more air than water goes up the uplift tube.
Look for an excessive build up of material on top of the sponge material or stir up some dirt near the water intake and see if it is being sucked in.
Poor water clarity for all filter types will also be an indicator of a filter starting to clog. A lot of other factors can cause this however such as overcrowding, too much light or overfeeding.
The best indicator that your filter is starting to lose efficiency is an increase in the ammonia level. Ammonia should always be at zero ppm (parts per million). If you add a lot more fish then you need to monitor the ammonia levels again.
If you sniff the water of an aquarium and can detect even the slightest sharpness you can bet the ammonia level is too high.
You may find that the water in a new aquarium starts to go cloudy after a few days. This is called a 'bacterial bloom'. More bacteria are present than is necessary. Continue to monitor the ammonia and you will find that the water eventually clears after a few days as the bacteria balance themselves to the amount of ammonia being produced.
When you clean out your filter, you want to leave as much of the bacteria colony undisturbed as possible.
If your filter has foam and/or hard material it needs to be cleaned in some of the aquarium water it has been filtering. This means the bacteria are not shocked by a change of temperature, pH or a sudden dose of chlorine from fresh tap water (chlorine is in tap water specifically to kill bacteria). Siphon out some water into a bucket for this purpose.
The filter media doesn't have to be cleaned spotlessly. If the filter is the correct size for the aquarium it will soon sort out any cloudiness caused by you disturbing the filter media.
After I have cleaned out a filter, I add a squirt of Stress Zyme to the aquarium as it helps the bacteria numbers recover more quickly. It is also very good for new aquarium setups as it speeds the build up of nitrifying bacteria in the filter.
The by-product (waste) from the filter is nitrates. It is unlikely you would have enough plants to absorb all the nitrate. This is where water changes come in. It is to reduce the amount of nitrates in the water.
Without partial changes the water will take on a yellow/brown tinge that no amount of filtering will remove.
Another reason is that although most fish tolerate very high concentrations of nitrates, some don't.
Most importantly, nitrate rich water encourages green algae.
Use your nitrate test kit to monitor the levels so you can adjust the number of water changes you need to do.
Keep the nitrate level at no more than 20-30 PPM (parts per million).
There are various special filter media you can put in your filter to achieve certain desired results. The most common is activated carbon (charcoal) which is put in for a short time to absorb perhaps medications that have been used, or to polish the water for extra clarity.
There are also various resins designed to absorb nitrates if you are unable to do regular water changes or want to continually strip nutrients out of the water as they are being produced.
The most important maintenance you do on your aquarium, after water changes, is filter maintenance. The best way to spend less time cleaning aquarium filters is by choosing the right one for the job.