When the competitor to Excel first came out the manufacturer reported on comparative test results between their product and Excel, following the instructions on the bottles. The competitor's product worked much better and didn't kill off the vals in the tank. But, this result was exactly what should have been expected because the competitor did not recommend the huge dosage at water changes, and their daily dosage was higher, as I recall. After reading that I interpreted the test results as showing that Excel should be dosed every day at the same dosage every day with no increase for water change days. And, the dosage should be around 2 ml per 10 gallons. That is what I have been doing and it works much better that way. I believe this is now the accepted "correct" way to dose Excel.
Since many of us used Excel at 5 ml per 10 gallons on water change days, and it didn't do obvious damage, except to vals and a few other sensitive plants, it is obvious that using Excel at that dosage every day, as a way to help kill off algae, will not harm the fish or plants. It does make it very expensive to use Excel, but if it kills off the algae it might be worth the cost.