I have a liner pond about 5ft * 3ft and at max about 18ins deep. 6 fish plus one lily some oxygenating plants also bulrushes and 3 marginals. There appears to be a dust like substance covering the plants and all lower surfaces of the pound. Using a commercial product called Sludge Buster, this sinks the dust but not completely it certainly does not eradicate the problem. This dust is easily disturbed. I have not got a external filter but have a small fountain to help circulate the water. What suggestions are there to resolve this problem? Regards.