Sludge in Garden Fish Ponds

We had a query today from the owner of a large pond. The volume was some 50,000 litres (10,000 gallons in US) and he was sick and tired of the sludge at the bottom of the pond. He wanted to know what type of filter he would need to remove the sludge and keep the bottom of the pond clean.

This query once again reminded me that the vast majority of pond keepers see a filter as having the job of removing solids. It is not surprising since filtering of anything is normally associated with removing solids from a liquid.

A BIOFILTER IS NOT A TRADITIONAL FILTER

In a pond environment we should always therefore speak of a biofilter and the most important job of a biofilter is NOT to remove solids but to purify the water in which the fish live.

In a pond where there is no biofilter many things can and do go wrong and the worst that can happen is that all the fish die from either poisoning or disease. The following is what's important to understand...

A fish, through its normal metabolic processes, actually pollutes the water in which it swims. The pollutants are nitrogen based chemicals in either ammonia or nitrite forms. The biofilter converts the ammonia and nitrites to nitrates which is relatively harmless to fish although nitrates are responsible for murky water and blanketweed ... refer to my previous submissions on the use of Viresco or visit this "all about blanketweed". page  or if you live in South Africa use this Viresco page.

A biofilter is used as a place to hold large colonies of bacteria which cling to surfaces inside the biofilter such as plastic tubes or balls, foam or other media such as Alfagrog . These bacteria remove the pollutants from the water as it is pumped through the filter. Now the filter will actually remove some solids but it will be a small percentage of the solids that find their way into a pond.

Solids in a pond come from wind borne debris, excess fish food, rain run-off and fish faeces. These solids normally settle quite quickly and are not picked up by the pump. As a result over time a sediment consisting of organic and inorganic debris builds up on the bottom of the pond and after years this can become sludge.

Viresco and other sludge digesters will to some extent control the organic debris but such additives will have no impact upon the inorganic (soil, sand etc) debris which will continue to build up.

For people with small ponds it is not too big of a job to empty the pond every year or so but if the pond is large then emptying the pond is no small job and not recommended.

BOTTOM DRAINS WORK

Every large pond should be built to include a bottom drain... such a system allows water to accumulate at the drain point where it is continuously removed with the recirculating water that is sucked from the bottom drain. Solids are then removed in a settling chamber of some description (a vortex unit is the most efficient). The settling chamber is then purged occasionally. This results in a pond with no debris build up.

POND VACUUMS

Of course this doesn't help if the pond has already been built which really only leaves one option other than emptying the pond and that is to use a specially designed and safe-to-use pond vacuum.

The best pond vacuum is made by Oase in Germany. In South Africa it's available from Super Pumps  where a much cheaper pond vacuum devices can also be seen.

In the UK you can buy at Bradshaws . In the USA try Fosters and Smith .