This question is frequently asked, but very difficult to answer.
As pond fish species vary greatly in size, behaviour, appetite and feeding habits, fish stocking should be viewed in terms of the total weight of fish in a pond, rather than the number of fish put into a particular pond. This is because a single large fish (e.g. Koi Carp) could be equivalent to multiple fish of a smaller species (Goldfish, Golden Orfe etc.) in terms of its feeding voracity and waste production amount. Put simply, a particular pond can support fewer Koi Carp than it can Goldfish for example, due to the vast size difference between the species.
If a pond’s fish stock is predominantly made up of one fish species, or a few different species of similar size potential and feeding habits (e.g. Goldfish, Shubunkins and Rudd), a specific number of fish can be settled upon by estimating how much one fish might weigh (keeping in mind that in the right environment the fish will grow), and multiplying up until the weight limit is reached, or the number of individual fish is enough, whichever comes first.
As Goldfish and Koi Carp are the most commonly found fish in garden ponds, and these species represent fish of differing sizes, these species can be used as an example from which a filtration circulation rate, UV power requirement and a sensible fish stocking density can be identified, using the pond type ‘Goldfish Pond’ or ‘Koi Pond’, depending on the dominant stocked species.
When it comes to weight of fish, a basic rule of thumb would be as follows:
For example: A pond of 4,000 litres could support up to 4kg of Goldfish, or 8kg of Koi Carp, provided the filtration system used is correctly specified.
These stocking densities assume that there is an adequate level of filtration on the pond to cope with the stock of fish. This means a suitably sized pond filter and a pond turnover rate of one complete pond volume circulation through the filter every 4 hours or higher (Goldfish pond), or every 2 hours or higher (Koi Carp pond).
Some typical Goldfish pond filter systems include the OASE Filtral UVC submersible filters and BioPress pressurised UV filters for smaller ponds up to around 5000 litres, FiltoClear pressurised UV filters, FiltoMatic and BioSmart flow-through filtration systems which can support Goldfish ponds up to around 18000 litres.
Some of these filter systems are also suitable for smaller Koi Carp ponds, but usually Koi ponds are of a higher volume and stocking density, and thus more demanding, so OASE BioTec ScreeMatic and auto-cleaning ProfiClear Premium systems are usually more appropriate for Koi pond filtration and larger goldfish ponds 20,000 litres plus.
Keep in mind that more fish puts more demand on the ecosystem as a whole, so a reserved approach to fish stocking is advised. It is quite easy to overstock a pond with all of bright and beautiful types of pond fish available, but a low-stocked can be just as pleasing without the demands that come with a high fish stock.
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