I've had good results with Algaefix even with higher dosage. I had some bushy clado for a long long time in my tank that I could never get rid of it so I finally gave in and used it. I dosed the recommended amount EVERYDAY for a week and there was no ill effects on the fish or the plants. The discus, otos, and rams were all fine, didn't lose a single one. The clado though was pretty much all dead by the third day, I just wanted to make sure I got it all.
A side note, doesn't dead/dying algae release ammonia? if you had a ton of dying/dead algae like green water would that not cause a spike and in turn kill your fish.