During Christmas time everybody is busy with christmas preparations.
Nevertheless you shouldn't forget to feed the fishes or even the regular filter maintenance.
Aquarium filters have three important tasks to perform:
1. Mechanical pre-filtering
2. Providing an optimal environmental for the filter bacteria that break down
ammonium and/or ammonia and nitrite (biological filtration)
3. Creating water movement
In evaluating a filter system it is often only the biological effectiveness of
the system that is taken into account. However, without mechanical prefiltering or regular cleaning of the filter the usual aquarium filter systems such as external canister filters, internal or hang-on filters, will never achieve their full biological breakdown capacity.
Biological filtration is performed by so-called autotrophic filter bacteria, known as nitrificants, which form a biofilm on the surface of the filter medium, where they generate the energy they require to live and
for growth solely via the breakdown of ammonium and nitrite to nitrate. In the majority of aquaria only a small amount of this nitrate is used by plants or bacteria that break down nitrate, and the bulk of it is removed by regular water changes. The bacteria require a huge amount of oxygen (4.5 g of oxygen for every gram of ammonium!) for the conversion of ammonium through to nitrate, a process known as nitrification, and as a rule they obtain this from the aquarium water flowing into the filter.
And this is precisely the reason why a good pre-filter and/or regular filter cleaning is so important. As over time fish excreta, mulm, and plant waste will accumulate in the filter, clogging the filter medium and hence also restricting the supply of oxygen to the filter bacteria. In addition the accumulation of organic mass in the filter encourages the growth of so-called heterotrophic bacteria species. These reproduce appreciably more rapidly than the filter bacteria and inhabit mainly the upper layers of the biofilm. If the latter becomes too thick then the underlying filter bacteria will no longer receive sufficient oxygen (see diagram). In the event of a heavy loading with organic mass (for example in rearing tanks with heavier feeding) the heterotrophic bacteria are even able to clog the filter medium through their own growth alone.
In heavily clogged filters anoxic (= without oxygen) zones may sometimes develop, inhabited by bacteria that break down nitrate. These species live in anoxic conditions and obtain their energy via the breakdown of nitrate (denitrification). But because there is not a total lack of oxygen in such clogged filter, and in addition these bacteria also require additional food in the form of organic carbon, the process of nitrate breakdown often remains incomplete, resulting in toxic nitrite being released into the aquarium. Hence special nitrate filters and filter materials are necessary for the denitrification process to be effective.
Hence the pre-filter plays a special, if not the most important, role in the aquarium filter. Its use, and the regular cleaning or changing of the pre-filter material,will keep the aquarium water free of suspended matter, ensure water movement and above all the requisite oxygen supply, and thereby make possible the provision of optimal conditions for biological filtration (nitrification). This applies above all to modern biofilter media composed of sintered materials, which offer the filter bacteria a particularly large surface
area, as their tiny interstices rapidly become clogged with dirt or unwanted
heterotrophic bacteria.
The ideal medium for the pre-filter is filter wool, which can be regularly renewed to avoid the trouble of cleaning it. This can be most readily achieved in external canister filters or chambered filters,where the wool is used as the first layer of filter medium. Of course, the use of fine-grade sponge is equally feasible, and this should be regularly cleaned. In such systems all that is then needed is regular checking of the biofilter material, which may not need to be removed or cleaned at all when used in combination with a well-maintained pre-filter!
Let us now apply our knowledge of the biological processes in the filter to systems in which only a single sponge is used (for example filter matting and many internal filters) and other measures are necessary. In order to remove dirt and unwanted bacteria the filter should be cleaned during regular water changes, if possible in a bucket of aquarium water, or in the case of filter matting regularly sucked clean with a siphon hose. It is essential to avoid cleaning in mains water that is too hot or too cold, as the sponge also contains the important filter bacteria and these will be unnecessarily decimated by such a procedure.
How complicated and time-consuming a filter is in terms of maintenance, depends not only on its size and construction but above all on the type of pre-filter. As unfortunately it is the latter that makes regular contact with the filter unavoidable.

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