I feel that some people clean canisters far too often. Everytime you clean them out you are removing a portion of the benificial bacteria colonies, sometimes creating a mini cycle as well. Obviously you don't want them so dirty they start producing nitrates, but cleaning them out too often can do harm as well. If you're doing every couple months IMO that is fine.
What Steve says above....
Cleaning often has a drawback, especially if the single filter. It can cause enough chronic water quality issues to affect fish in the long term...
Prevention is the key.
I use pre-filter sponges on all intakes and I clean those each week. I do not touch the filters often, and when I do, it is one at a time. I always keep more than one filter on a tank except for too small tanks. In my experience using pre-filters, the main filter media stays clean for a really long time and the impeller stays spotless as well...so filter flow flow is never reduced...Thus also oxygen supply is not compromised.
It is imperative that the media inside the bio-filter stays clean in order to be operating at optimum in terms of the efficiency of bacteria, which is dependent on the supply of oxygen, which in turn leads to the right chemical processes going on.
Dirty filters are oxygen deficient. Oxygen deficient bacteria does not do nitrification very well. The ones that do "nitrification" in low oxygen levels are facultative heterotrophs but they're like the donkeys of the racing world. There are always some in the race but they won't ever reach the finish line on time, although they'll run some distance for your money....
Nitrification is the process of oxidizing ammonia and nitrite to nitrate, the first two of which are abundant in a fish tank but oxygen isn't...And the archaea/bacteria that does nitrification is fully aerobic... It dies in anaerobic conditions caused by clogging the filter with poop and other detritus. So it isn't the nitrate from the poop that made your fish sick or worse, killed your fish...It is the inefficiency of the bio-filtration leading to poor water quality...a lot to do with ammonia and nitrites, low oxygen levels, and very little to do with nitrates...
Oxygen deficient filters can become partial de-nitrifiers, meaning they'd be reversing the nitrification process back, the ideal conditions being high nitrate levels, high organic levels and low oxygen...But being in the filter, that process gets disturbed all the time, so one ends up with chronic nitrites instead of nitrogen gas..... In scientific studies, elevated nitrates are always associated with elevated nitrites and the reason is denitrification...The symptoms of nitrite toxicity in fish is what people tend to associate with nitrate toxicity...Denitrification, although it may sound as a good thing to some on paper, is something one never wants in a filter. It does happen in a healthy undisturbed substrate at the bottom layers...but the bi-products stay inside the substrate...for the most part...
There are two main forms of organic build up in a dirty tank, urea, which is a solid that dissolves in water, and fish poop, the only solid we tend to see and think about.... Urea ends up in the water column so there's no other way but remove it via water changes. Besides the ammonia produced directly by the fish, urea is the main problem, not the poop, as urea and urea decomposition happens in the water column and it's where the bi-product of urea decomposition is released, i.e. ammonia.
Poop can safely be decomposed in the substrate where it's bi-products are either locked, unless someone disturbs the substrate, or they're taken up by plants, if one keeps a planted tank, all without any of it reaching the fish. It is not that wise to be stirring your substrate that often either for the same reason. Do an ammonia test after "a good cleaning" of the substrate and you'll know why....Alternatively in a well oxygenated tank and substrate(via good linear flow or plant roots or both), there are high levels of nitrification in the substrate itself. Hence bare tanks and non-planted tanks are at a big disadvantage unless one cleans a lot...Regardless, poop should be removed too as it also needs oxygen to decompose and oxygen in a tank is of short supply. The least organics for a tank to decompose, the better...
The short story of all that is, if you keep a bare tank, it's important that it's kept really clean as it has no safety factor besides the owner's habits.
In a planted tank with substrate siphoning is of no big importance but clean filters and large water changes are, for any type of setup.
Additionally, flow and surface agitation are important for oxygen levels. Overstocked tanks are prone to low oxygen levels. Bare overstocked tanks are a ticking bomb....very hard to maintain in a healthy state even with one's best intentions...
