So you have pressurized CO2 AND you dose excel?
I suggest dosing EI personally excessive nutrients do not lead to algae growth in and of themselves. Other factors come into play when figuring out what is causing an algae bloom, light being a biggie, CO2 being another. According to your OP your not dosing PO4 add the KHPO4 to your dosing regime and see if that helps. How long of a photo period do you have?
Dose CSM+B, K2SO4, KHPO4, & KNO3 if your nitrAtes are below 10 to 20 ppm.
Here's a good write up on the EI method or fert dosing.
And some recipes.