One of the questions we are frequently asked by our customers is about safe stocking levels for a pond. To achieve a safe stocking level you need to be aware that fish consume something known as ‘dissolved oxygen’ from the water around them, and that they excrete waste products that can have a negative impact on the water quality within your pond.

When we look to keep a large number of fish in a pond of a relatively small volume there are always going to be strains on the water quality as a consequence. It is still up for debate really on how much weight any pond should contain – this is called the stocking density. It was always believed before modern filtration, that dissolved oxygen levels in a pond were the most important factor that influenced the amount of fish within a pond, the main source of oxygen used to be through the surface area of the water that was exposed to the atmosphere. The larger the pond surface area equals a larger transfer of dissolved oxygen.

Most modern koi filters have a much higher surface area media and aeration that is fed directly to them, which again interferes with any way of providing an exact calculation of the number of fish people should keep.

Does that mean we can’t advise on it? Well, not really, as there are some rules of thumb. Sticking to these should allow for healthy pond management. There are quite a few different guidelines on this, but we like to stick to the one used by many a fish keeper for years and this is that every 2.5cm of a fish’s length requires a surface area of 0.09m², to allow for the required amount of dissolved oxygen to remain in your pond.

Can I keep more fish if I have more bio-media in my filter?

Well, hundreds of hours and millions of pounds have gone into the study of calculating the required surface area of bio-media to sustain a certain population of fish. Research here is trying to ascertain if there is a certain amount of food that can be fed to fish, in where a certain amount of bio-media can accommodate the required population of microorganisms to break it down. This is all a little bit impractical for the modern fish keeper though, as it’s very much confined to lab testing. It all sounds very complicated, but to keep it simple, most filter manufacturers will work on the rule that for every 1kg of food that we put into a pond per day, you need 200m² of bio-media surface area to safely maintain this. To adopt this rule you will need to weigh the amount of food you feed in a day and make sure you increase the media if necessary or reduce the amount of fish your pond holds.

Stocking density?

OATA (The Ornamental Aquatic Trade Association) have released figures for aquatic shops on stocking levels and they have stated that Aquatic Retailers should keep no more than 8kg of fish per 220 gallons (or 1,000 litres). However we would advise customers that 2kg per 220 gallons or 1,000 litres is more accurate. To stick to this criterion you would need to first work out the volume of your pond (link to pond calculator) and then weigh your fish.

So how much food can I feed?

This is where it can get a bit complicated, here is the recommended calculation:

Using the above stocking density then, you can accurately use the ‘1kg of food to 200m² of bio-media to surface area,’ and we will use the 2kg of fish per 220 gallons. Fish should be fed 2% of their own body weight per day as a maximum.

A 2,200 gallons (10m³) pond would hold a maximum of 20kg of fish (2200 ÷ 220 x 2) which should be fed a maximum daily food ration of 0.4kg (20 x 0.02). If 1kg food requires 200m² of Biological surface area, then this daily food ration would require bio-media of 80m² of surface area.

So, in summary the three big general rules of thumb are:

1: Fish size - 2.5cm of fish per 0.09m² of surface area

2: Fish weight - 2kg of fish per 220 gallons (1000 litres)

3: Feeding - 1kg of food per 200m² of Biomedia surface area

Why is this important to me?

Filtration now allows us to keep way more fish than nature intended, this can come at a cost to water quality within your pond. We can add an air pump to bump up the dissolved oxygen levels, so the only real limitations lie solely on your filtration and your keeping an eye on everything via water testing. Filters have to breakdown nitrogen-based products that fish produce, all fish produce these as part of their metabolic process, but left unfiltered and allowed to accumulate these will likely lead to fish deaths within your pond. These ideally need to be broken down and removed by your pond’s biological filtration. The only other way to dilute this is via water changes, it isn’t accurate though and using de-chlorinators all the time would work out costly. It is true though that ponds do benefit from water changes and changing around 10% of the water once a month is recommended. Rather than using a lot of de-chlorinating treatments when you do this, check out our inline de-chlorinators which are a brilliant bit of equipment to have.

What to do if you are overstocked?

There are a few things you can do if your pond is overstocked and your fish health and/or water quality is suffering as a result:

Reduce feeding, and ensure you’re using a good quality food, correct for the time of year.

Clean out your filters using a bucket of pond water, or water from a water butt.

Monitor the water quality.

Add aeration to the pond.

Carry out a partial water change if needed.

I have always been an advocate of having an understocked pond, it gives fish more room to grow and in the long run is much simpler and easier for you the hobbyist. So before you buy your next fish have a think, do I need it?