When you say "overdose excel", what dosage is that?
I am wondering the same on the excel
Use hydrogen peroxide instead. It has the same effect and it is SIGNIFICANTLY cheaper. Start with about 2ml per gallon of tank water. Altho I have to assume there is a point when this formula is not beneficial like using 100 ml for one spot in a fifty gal. You should be using a ml or 2 on each affected area.