sand filters are bad For koi ponds
Sand filters are bad for ponds but great for swimming pools and can only remotely be justified as solid removers. They perform little if any nitrification - poor surface area contact, low levels of turbulence, dead areas for pathogenic bacteria to accumulate, and low oxygen levels. I strongly believe there is no place for a sand filter in a pond environment. They are also extremely costly to run. The following is an extract from the Aquaculture Dictionary to back up my personal views on sand filters. A far better alternative to sand filter can be found in the bead filter here
This extract is actually discussing the commercial use in aquaculture of sand filters but the principles remain for your consideration and comment. By the way algae can be as small as 4 microns and this is why a sand filter does not remove algae. Here is the extract:
Sand Filters
"Used for the mechanical filtration of solids, not to be confused with fluidised sand filters, which are used for biological filtration. Although the size of particle that the sand filters remove is dependant on factors such as the size of the sand particles, the depth of the bed of sand and the flow rate through the bed, sand filters are usually regarded to filter water to a nominal 10 microns.
Sand filters are designed in two distinct ways:
The first is a simple box structure that operates with a low pressure across the filter. The water flow-rate to cross sectional area ratio of such filters is low, and the filters tend to rapidly block in the first few centimetres, with the rest of the filter staying clean. Such filters are only of use in applications where the use of pressure filters is impossible or the water is generally clean and there are only a few particles that need removing. Such an example may be a ground water supply which is thought to be contaminated with pathogens through seepage into the spring / borehole. Large filters of this design are difficult to clean effectively, usually resulting in the bed being periodically dug out and replaced by fresh sand.
Pressurised sand filters are in common use in many aquaculture applications. They consist of an enclosed vessel that is typically half to two thirds full with sand. Water is pumped into the top of the filter under a pressure of approximately 1-2 bar and is forced through the sand to a water-collecting device at the bottom that allows the water through, but not the sand particles. The flow is then reversed to back flush the filters. Pressurised sand filters are expensive to use for high flows due to the cost of pumping the water through them. They are however used extensively in hatcheries and also some recirculation systems, where they are either plumbed in for all the water or as a side stream, where only a percentage of the water flows through.
Their limitations in recirculation systems is that, in addition to the operational costs, they use a lot of water for back flushing (a typical sand filter in a recirculation system will require back flushing 4-6 times a day for 5 minutes each time. The water flow rate whilst back flushing is the same as the flow rate when filtering). This is exacerbated by the fact that sand filters in re-circulated water will also act as biological filters, and a layer of heterotrophic and nitrification bacteria will build up on the sand, causing channelling and increase back washing frequency. The back wash process is insufficient to eliminate all the bacteria that soon multiply and block the filter again. A way round this is to add ozone or other disinfectant chemicals to the water when back flushing (the advantage of ozone here is that any residual amounts after back flushing will quickly be neutralised by the organic compounds in the water). Now that self-cleaning mechanical screen filters are available with screens of less than 10 microns, the use of sand filters is becoming less common. "
For small to medium ponds where you wnat a suitable pressure filter look here
Hozelock
BioForce UVC Pond Filters make hiding filter easy
Large Koi Ponds Ideal Bio Filter That Runs Almost Automatically ... the bead filter is preferred