Join Date: Mar 2011
Location: Oregon State Beavers!
Given the wpg you supplied I'm assuming you are using 2 48" T5HO light bulbs? That is way too much light for your needs. Without CO2 being supplied I wouldn't even be using one of those. The WPG rule is totally useless and certainly should NEVER NEVER be applied to HO lighting. I would get rid of that light or raise it up off the tank quite a bit. The current trend is showing that 2 T5HO bulbs with individual reflectors is high lighting for a 75 gallon tank that has CO2 supplementation and fertilizing. Depending on various factors, tanks typically see about 6-10 hours of light per day.
Also, it is not an over abundance of nutrients that is causing your algae (which I can't really see in the picture so I can tell what kind it is). Really it looks like nothing more than your everyday bit of green algae that develops throughout tanks. Too much light, not enough CO2, that is what is causing algae. People have been successfully dosing their aquariums via the EI Method and that has produced tremendous results with little to no algae.