Ideally the filter needs 3 parts. First an easily removable/cleanable fine mesh to take out the lumps, then a zone where the water can flow through a biological support media where bacterial growth will remove the dissolved mater, and finally a settling chamber where clumps of spent bacteria will drop out. You can build the whole thing in a 50 gallon water tank. It needs a internal partition to separate the bio media from the settling chamber, and the first filtration can be a pad of plastic foam. Make sure the outlets and overflows discharge back to the pond to avoid water loss. You might consider a UV in the dirty water feed. Ideally you want to have a pumping rate of something like 200 gallons per hour
