Pond Sand Filters

Some fish keepers use a sand filter as a final stage of there water filtration system. The water is pressurized to 0.85 - 1kg/cm2 ( 12 - 14 ib/in2 ) as it passes into the filter, where it is filtered through sand contained inside a pressure cylinder.
The sand filter removes the most finest of particles from the water so that on return to the pond the water is almost entirely free from any solids. There is a great deal of evidence that bacterial activity takes place inside the sand filter to further the breakdown of toxic wastes produced by fish. Without doubt sand filters are a very beneficial addition to a fish keepers filtration system.
On rare occaisions however, a sand filter can draw air into the pond water, when this happens, the air is forced by the pressure of the sand filter into solution in the water, leading to whats called supersaturation. The supersaturated water builds up in the pond creating a fizzy apperance and coating the surface, plants and anything else in the pond with a fine layer of bubbles. This can lead to a clinical disorder known as gas bubble disease, in which gas bubbles form on the fin bases, as well as in the blood and amoung internal organs ( known in the diving community as the bends).
Should such a situation occur when using a sand filter you should immediately by pass that part of the filtration system and seek advice from the manufacturer. The excess air in the pond will gas off over some hours as the water passes through the main filters and aeration system. Fish suffering from gas bubble disease may take several weeks to recover completely as they naturally decompress.