Aquarium filters must be changed and cleaned regularly. Not paying any attention to your filters can result in poor quality water.
Changing Filter Medium
You must change the medium in your filter on a regular basis. Are you using activated carbon? Change it every 3-4 weeks. Are you using other filter media? Clean it or change it every 2-4 weeks.
It is important to always make sure that you filtration system is actually filtering and cleaning water, not serving as a harbor for chemicals and problems.
Sponges & Pads
Sponges and pads are typically used to trap debris and larger particles in the filtration system. Changing or cleaning these sponges and pads is essential. The debris that builds up on these filtration materials will eventually begin to leach toxins into the water, which is not a good thing.