You are doing everything right. The only problem is a phosphate reading of 1000. This is still a very high number and unfortunately will still cause algae problems. My answer to this is straightforward: you HAVE to eliminate the phosphates. The reading ideally needs to be 0, but if it's under 100, it should be fine. It's much easier just to add some phosphate remover. When I do this, though, I always know that the filter is going to need to be cleaned within a few days.