I recently tried phosguard in my 200 gallon tank, and it lowered it from .6 to .4mg/l . That was even a questionable change. I'm going to try a phosphate sponge next.
Ken't phosphate sponge is the boss. Why are you getting phosphates anyway. Try looking at the source also. It is usually overfeeding flake food and using bad water. Always use RO/DI water.
If I'm using ro/di water and use Marineland Black Diamond carbon (which I heard was the best), wouldn't the only problem for high phosphates be overfeeding? I don't feel I'm overfeeding. I only feed once daily and I feel it's a small amount.