Is the phosphate also in your tap water? Test some other water with your test kit that you know isn't high phosphate....it could be faulty. Test kits are really unreliable. I just had to throw out a nitrate test kit when it gave me three totally different readings on the same water.

High phosphate won't necessarily cause algae. I dose lots of phosphate in my tanks, I don't have a test kit so I just add what amounts to probably too much, plus whatever is in my tank/tap water besides that, and I've never been able to induce algae that way.