I had a phosphate problem about 6 weeks ago, and I'm still working on getting rid of it. Luckily, I'm pretty much done.
In my tank, what would happen was this:
I had explosive algae growth after topping off a couple of times w/ regular tap water. I know this was bad, but the roads were icy and I couldn't get to LFS for DI water. I now keep 5 gallons extra on hand.
Anyway, so my tank was literally FILLED with red algae. I brought a picture to my LFS and they said it was the worst they've ever seen. The told me to use Chemi-Clean to bring my phosphates down. I dosed it with the stuff a few times, and everything was cool. Algae was going down, phosphates were going down, and the tank was returning to normal. Then I had another algae explosion. Apparently, decaying algae gives off phosphates, which in turn feeds more algae. The best thing to do that I've found is to do water changes twice as frequently, making sure to remove as much algae as you can, and dosing regularly w/ a phosphate reducer. I also found that it was helpful, if you have red algae, to peel it off the rocks (the less that decays in your tank, the less phosphates it'll add).
Anyway, you didn't say anthing about algae, so I hope your problem isn't as bad as mine was. Good luck!
btw, Chemi-Clean is VERY expensive for the amount of chemical you get (about $18 for just a few grams), but it doesn't take much at all per dose. One bottle lasted about 20 doses in my 20gal. Just don't knock the bottle into the floor, like I did.
