I agree with 'morval'. You can use one of 3 products which are really good. Phosban, Phos-guard or Phosphate Sponge. I used Phosphate Sponge. Its a granual media that you put into a mesh filter net. You then drop this ontop of your filter pad. The initial charge (<48hrs) removes Phosphate, then you need to dump it out and replace with new, you then leave this in for about a week or so, this will remove any Silica. It's pretty easy and works.