You did not say if you were using tap water or ro/di. How much new rock was introduced into the system. How long has your 120 been set up. don't think it is the IO many people have used it for years to include myself with no ill effects. If you added a lot of new rock the system might still be in a cycle even if your readings are were they need to be. algae comes and goes away only to be replaced with a different algae that to will go away. But phosphate is a factor in this. if phosguard or similar is not helping quick enough you can buy a phosphate sponge to put in a canister filter to remove it faster for short term use.