Just the other side of this coin. I have used Excel in multiple tanks now for about 14 years. This includes tanks with discus, Altum angels, zebra plecos and a host of more mundane fish and had no problems. In tanks where I have been using it regularly I have had well over a 1,000 fry do just fine over the years. Some of my clown loaches have been with me since 2002 and have been in a planted tank which is dosed with Excel every week.
However, I do not overdose this product and would never ever try to use it as an algaecide. I am well aware of the potential for this chemical to be toxic if misused. But then people use ferts which contain ammonia and nitrate, both of which can be toxic to fish. The issue is always one of concentration.
I also respect Byron's (and anybody else's) choice not to use it. I am the same way about products going into the water which contain aloe vera. Each of us has to make our own decisions about certain products and to use them or not.