Generally speaking the pond volume should be turned over in less than two hours.

However, the faster the better. The economics of this fact are the limiting factors. Pumps cost money to run.

There are enormous benefits to be gained by a faster turnover rate. There will be more oxygen dissolved into the water. Actually air not only oxygen. The solids will be removed quicker resulting in a clear pond. The fish impurities that are dissolved in the water will be removed faster. Your water will sparkle. Your fish will grow faster (depending on several factors such as temp. etc). The colours on your koi will, generally speaking, be better.

Contrary to this fact of faster turnover rates is another fact - the bacteria, and the many other microorganisms we know nothing or very little about, in the biological filter need TIME to convert the ammonia to nitrite and then to nitrate. If the water moves rapidly past the filter media a high percentage of the ammonia is returned to the pond.

You will then always have an ambient level of ammonia present in the pond water. Whilst it may not kill your koi it will certainly cause stress.(Stress = disease). The ambient level of ammonia in a pond is a reality. Your koi are excreting metabolic wastes all the time. Whilst the filter bacteria in the media is removing the ammonia the fish are still polluting the pond water. The larger the stocking densities the more of a problem this becomes.

As you increase the turnover rate the ambient level of ammonia decreases and at around a full pond turnover rate every two hours through the filter or filters the ammonia levels should be below what can be registered on a hobbyists test kit.

In order to get the water to SLOW down in the filter chamber/chambers but still maintain a high pond turnover rate the simplest thing to do is to increase the size of the filter chamber/chambers. i.e. to increase the surface area of the chamber. When this happens the velocity of the water slows down.

Yes, you can work back from velocity of so many inches per minute BUT if you have to construct a filter chamber you have to know the dimensions before construction begins.

Some microbiologists suggest a contact time in the filter chamber of around 15 minutes. (40 liters per square meter of surface area PER chamber - about 10 gallons per 10 sq. ft of filter chamber area).

As a rule of thumb a flow rate of between 60 - 80 liters per square meter of chamber surface area will slow the water down sufficiently to give 100% conversion of ammonia in a single pass through the filter chamber but still maintain the high pond turn over rate you were hoping for. A slow flow rate such as this will also encourage settlement of solids. (60 liters = about 15 gallons / 80 liters = about 21 gallons).

As a working example. Pond volume about 5000 gallons. Pump needed to turn the water over in under two hours = about 44 gallons per minute flow rate (unrestricted). Filter chamber surface area to cope with a flow rate of 44 gallons per minute = about 22 - 29 sq. ft. (15 - 21 gallons per minute per 10 square ft)

I hope I have my conversions correct.

The filter media should also be shallow to prevent blocking and to avoid deep regions suffering from a lack of oxygen.

You don't have to dynamite your existing filter if it does not fall within these parameters - if your fish are alive and well and growing you are doing something right.

You can also make some clever 'adjustments' or minor modifications to your existing filters to achieve these parameters.

Regards, Chris Neaves

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Last modified on Tuesday February 15, 2000