If my phosphate was that high I would consider using lanthium chloride to knock it down to a level that your GFO isn't getting as exhausted so quickly.
BE CAREFUL with lanthium chloride..it is very effective..and if you dose to much at once, it can be harmfully effective.
BE CAREFUL with lanthium chloride..it is very effective..and if you dose to much at once, it can be harmfully effective.