How do treat my water garden for too much phosphate/phospherous?

I have a 1200 gal water garden and i have done a 40% water change three times in the last 30 days to rid some of the phosphate...to no avail. The levels are way too high still, no plants will grow and i only have about 15-20 fish in there. 3 largest being about 8 inches, all others are their babies and are about 3 inches long. I can't put the phos-x media bags in my filter because i have a cannister filter that's in the ground, no media bag will fit in there...i can't figure where this is coming from. I have tap water that is treated when put in the garden.....anyone know what i can do or if there is a liquid product i can use to combat this? any help would be appreciated..thanks, AprilHow do treat my water garden for too much phosphate/phospherous?I would check for any runoff from neighbors lawn service. Check the ph with a garden center specializing in water garden.for recommendations or any specialty center. somewhere there must be a web site for a company selling the chemicals to correct your problemHow do treat my water garden for too much phosphate/phospherous?It's usually lawn fertilizer that causes that.