How to Assess Weight of Koi in a Pond and Implications on Pond Treatments, Stocking Rates and Feeding

John McLauchlan, Managing Director of Viresco (UK) Ltd, talks about his new formula for assessing the weight of koi and it’s implications on pond treatments, stocking rates and feeding… when it relates to the amounts of ammonia excreted by the fish.

John McLauchlan is the Managing Director of Viresco (UK) Ltd, a family run business based in Thirsk, North Yorkshire. The company sells specialist products into the horticulture and aquatic markets. VirescoTM Aqua and VirescoTM Koi are its microbial-based products for removing nitrate and suppressing blanketweed or other algae in ponds.

Click here for the Viresco  blanket weed and pond algae solution, pack sizes and prices worldwide (everybody pays the same delivered price) and to order online or if you prefer

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Contact Viresco directly to ask a question or to order using phone, fax or email

The issue of weight of koi in a pond

For a number of years, our company has been selling various microbialbased products used primarily to remove nitrate in pond and aquarium water. This results in the removal of blanketweed and algae bloom from the water. Once the nitrate level is taken down to zero, the various forms of algae die of starvation.

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The first product, VirescoTM Aqua, was launched just over nine years ago. From the outset, our main microbiologist adviser suggested that a 10g pack of VirescoTM would normally be sufficient to treat a 6,000-gallon pond.

Whilst we have had a great many successes with this one rate of use in general garden ponds and specialist koi ponds, we have questioned whether this simple application rate can be appropriate for all ponds.

Because the microbes in our mixes will remove all the nitrate arising from the ammonia excrement from fish, it seems sensible to assume that fish stocking levels, feeding regimes and types of food must affect the amount of ammonia being excreted by the fish and therefore the amount of nitrate coming out of the filter and into the pondwater.

We consequently suggested to koi keepers they should use more VirescoTM if their ponds contain more than 60in of fish per 1,000 gallons, if they are over-feeding and if they are feeding long runs of high protein food. Of these variables, we realise that looking at the size of fish in terms of length is rather weak when it relates to the amounts of ammonia excreted by the fish. Therefore, the purpose of this article is to examine more closely the relationships between koi length, their shape, their weight, their feeding regimes and their stocking levels.

This will enable koi-keepers to more accurately determine the amount of VirescoTM algae-removing products they should use in their pond, and indeed other products which depend on stocking rate information.

The key to this question is the calculation of the total weight of all the koi in the pond. Knowing the weight of the fish, the koi-keeper can also determine the amount of food he should be giving.

In the UK, the volumes of hobby ponds are usually measured in gallons. Fish food is sold in kilos. The lengths of fish are generally known and given in inches. We sell our pond VirescoTM products, VirescoTM Aqua and VirescoTM Koi – in grams for various pond volumes, expressed in gallons.

There is thus a mixture of metric and UK units are used by the pond-keeper. In this article, we shall link the length of a koi in inches to its weight in kilos! There is a particular advantage in doing this as will be seen in the section below.

Weight of koi versus length

The weight of a koi of a fixed length is also related to its shape. Three distinct shapes are considered. These are 'standard', 'fat' and 'skinny'. From various sources that give the weights of koi relative to their length, we have been able to devise a simple but relatively accurate formula to calculate their weight from their length.

The weights of koi are directly proportional to the cube of their length. The approximate weight of a koi can be calculated from the simple formula, W= L^3/K, where W is its weight, L is its length and K is a constant that depends on the shape of the fish.

For a 'standard'-shaped koi, its approximate weight can be calculated from the following formula: W= L3/5000 where W is the weight in kilos and L is the length in inches.

Fatter koi of the same length can be up to 50% greater in weight than the standard-shaped koi. A standard-shaped koi can be as much a double the weight of a skinny-shaped koi. Thus the formulae for the fat and skinny koi shapes are as follows:

W= L3/3333 (fat-shaped fish)

W= L3/10000 (skinny-shaped fish)

It can be seen that using inches as the unit for length and kilos as the unit for weight, the three different constants for shape are relatively easy to remember – 3,333, 5,000 and 10,000. (fat, standard and skinny).

Thus, for 36in of fish, one fat one at 36in length weighs about 14.00kg and six skinny skinny-shaped koi, each at 6in length, weigh a total of about 0.13kg. Thus, the one fat one is over 100 times heavier than the sum of the weights of the six smaller ones. Some examples are displayed in the table below.



Stocking levels

Fish stocking level is a subject that should concern many more pond-keepers than it seems to at present. Many do not realise their fish can put on considerable increases of weight in a season. Some also add more fish to their pond and the filter, which during the previous year had more than sufficient capacity to turn all the excreted ammonia into nitrite and then nitrate, is now working near its limit.

1kg of fish per 20 UK gallons would be classed as a very high density koi pond

1kg of fish per 200 UK gallons would be classed as a medium density koi pond

1kg of fish per 2000 UK gallons would be classed as a low density koi pond

Use of VirescoTM Aqua and VirescoTM Koi in ponds

As mentioned above, we suggest that 10g of either VirescoTM Aqua or VirescoTM Koi be used in 6,000 gallons of pondwater.

These two products remove nitrate that arises from the filter from ammoniacal fish waste. The implication is that it refers to about 21/2kg of koi weight per 1,000 gallons, i.e. about half the stocking level of the medium density pond of 5kg per 1,000 gallons shown below.

This means that 10g product can be used in a 6,000-gallon pond with up to 15kg of fish in it.

This 15kg can consist of, for example: Two x fat 30in koi or Four x fat 24in koi or Nine x fat 18in koi or Three x standard 30in koi or Six x standard 24in koi or Thirteen x standard 18in koi or Six x skinny 30in koi or Eleven x skinny 24in koi or Twenty six x skinny 18in koi.

For any weight greater than about 21/2kg per 1,000 gallons, more VirescoTM should be used proportionate to the increased weight. The amount of VirescoTM recommended for use relates to a maximum feeding regime of 2% of body weight, i.e. for good general growth. If overfeeding takes place, e.g. at the rate of 3% of body weight, then 50% more VirescoTM should be added.

Similarly, if high protein food is used, then 50% more VirescoTM should be applied.

Feeding regimes

 It is recommended the approximate food consumption for koi per day should be as follows: •

0.5% of body weight for minimum maintenance.

1% of body weight for better maintenance.

2% of body weight for general good growth.

3% of body weight for maximum growth.

Thus, the koi-keeper, knowing the lengths and the shape of each of his fish, can now calculate the total weight of all the fish in his pond. From that weight, he can then calculate accurately how much feed to use, dependent on the level of maintenance or growth he is seeking. Bear in mind that high protein food contains approximately 50% more protein than wheatgerm. (say, 45% v 30% protein).

Fish stocking level is a subject that should concern many more pond-keepers than it apparently does at present.

An example of one trial protocol

One very extreme trial situation was put to us. The relevant parameters were:

A 2,200 gallon pond, containing 15 fish each of 60cm (24in) length, with excessive feeding of 400g per day of very high (52%) protein food.

Assuming the 15 koi are all fat shaped, at 24in, each will weigh about 4.1kg. The total is, say, 60kg.

For 2,200 gallons, our recommended stocking level would be a maximum of 21/2kg per 1,000 gallons, ie about 5.5kg, when using VirescoTM at the rate of 10g per 6,000 gallons. The total fish weight is therefore about 11 times greater than that for our recommended rate of product use. The feeding rate is 400g per day. This is about 7% of the total weight of the fish which works out to be about 3.5 times our suggested rate of 2% of fish weight per day. The protein level at 52% is very high and is about 1.7 times that of wheatgerm. Thus the amount of VirescoTM that should be applied is 11 x 3.5 x 1.7 = about 65 times our recommended rate!

If the koi were not a fat shape, then this figure of 65 times could be reduced to about 43 times if a standard shape, or 22 times if a skinny shape.

This trial situation is very extreme and the hobby koi-keeper will never be running his pond with similar parameters.

We have stated above, for a medium to low koi pond stocking density of 21/2kg koi per 1,000 gallons, a minimum dose of 10g per 6,000 gallons of our microbial VirescoTM would be required. However, where fish stocking levels are much lower than 21/2kg koi per 1,000 gallons, e.g. typical fishing ponds, then considerably less of our product can be used per unit volume of water.

Viresco (UK) Ltd is now looking at the use of their microbial nitrate removing products in large fishing ponds. In the analysis given above, it is assumed that the entire nitrate in a koi pond arises from the conversion of ammoniacal fish excrement to nitrite and then to nitrate in the filter. However, additional nitrate can arise from the excrement of visiting birds and other pond creatures, such as newts or frogs. It can also arise from the additions of tap water, rainwater and other water inflows such as streams or springs.

Conclusion

Using the graph shown or the formulae quoted in this article, the koi-keeper can accurately determine the weights of each fish in his pond from its weight and shape. From this information, more accurate pond additions, e.g. food or our nitrate removing microbial products, can be calculated. Knowing the weight of the fish, the koikeeper can also determine the amount of food he should be feeding  

Click here for the Viresco  blanket weed solution, pack sizes and prices worldwide (everybody pays the same delivered price) and to order online or if you prefer

Contact Viresco directly to ask a question or to order using phone, fax or email