Is it green water? If so, a 3 day blackout tends to help. First do an 80% waterchange, and don't add ANY nutrients to the tank. Cover the tank 100% with a dark sheet, and tuck it all in so there is NO light seep. Peek in the corner and check the tank is PITCH black, not just dim, but pitch black. Turn off CO2. Then DON'T peek again for 36 hours (or you go back to 0 hours). No feeding, NO PEEKING. After 36-40 hours uncover the tank, and do another 80% waterchange. You may have to do a few more 80% waterchanges in the next week to get a real good foothold on it. They'll be fine. I just did this to my tank for BGA. It helped A LOT.
Do you dose fertilizers or any other additives? If so, what kind and how much? What kind of lighting? How long is the photoperiod?
Here is an AWESOME fw algae info website:
http://www.theplantedtank.co.uk/algae.htm