The most important thing when choosing a pond filter is the correct dimensioning. Pond filters that are too small are worse than no filter at all. You can read in detail in our article how to correctly calculate a pond filter and what you have to pay attention to.
Larger ponds with fish need a pond filter
Pond filter and pond volume
In principle, you can use the pond volume for the calculation. However, this is not always the sole decisive criterion. How much pond volume you have in your pond plays a minor role in the design of the filter - it's all about how much dirt you want to filter out of the water.
The type of pond is therefore particularly important for the selection of the filter. You usually don't need a filter for a natural pond - plankton and microorganisms in the water ensure a sufficiently good water quality and prevent algae from growing in the pond. The plants growing in the pond then also convert the dust and the drifts into the pond.
With a fish pond it is different. By adding fish food (for many types of fish you have to add additional food, such as koi carp) you fundamentally change the nutrient situation in the pond.
processes in the fish pond
In ponds inhabited by fish, a lot of ammonium is released into the water - on the one hand via the fish's gills, on the other hand there is also the natural ammonium content, which is caused by decaying plant remains in the pond.
However, if the ammonium content is too high, it can quickly kill the fish. The same applies to phosphates, which are mainly introduced through the fish feed. Here, too, an excessive content is harmful because it leads to increased growth of algae, for which phosphate is an excellent fertilizer.
Filter surface as calculation basis
The actual appropriate filter size for a fish pond is obtained by calculating the surface area that can be colonized by nitrifying (nitrate converting bacteria).
The basic rule: A specific filter surface area of 4 m² is required to convert 1 g of ammonium.
This means: when you bring in 100 g of feed, around 4 g of ammonium are produced (depending also on the fish stock) - and a filter area of 16 m² is therefore required. From the size of the required filter area, you can then calculate which filter volume is appropriate. Here you should then round up again, then you are on the safe side in terms of the filter size. With regard to the filter volume, despite the approximate calculation, always seek detailed advice from a specialist and provide realistic information for the calculation.
tips
The filter performance can of course be significantly lower with less ammonium input. With a natural pond you don't need a filter because there is no artificial entry. At best, you can actually use the pond volume in these cases.