[Pavel Chaloupka]

I would go with higher PPI(pores per inch), as you get much bigger specific surface to be colonized with all the helpfull microorganisms. The finer the material the better as long as you are able to maintain the flow of the water. If I recall correctly I one kubic meter of 20 PPI phoam has more then 2 square kilometers of the surface. If the flow of the water is not to fast, these foams almost never get obstructed. Of course it has to be polyurethan foam with open pores.