since this thread isn't very on-topic, I'll clarify:
Excel is a carbon supplement used in aquariums and comes in the form of an aldehyde (gluteraldehyde maybe?), it excess amounts in can keep fish from respirating properly and cause them to suffocate. (You aren't using this)
I have seen potassium dosed to 80ppm in tanks, and fish will not absorb much of this (you won't be dosing this much) so it won't hurt them. (You are using this)
Excess DIETARY Iron (not chelated or ferric forms) can cause stunted growth and liver toxicity. You are dosing the water column, again not a threat (you are using this).
Even if you dose double or triple amounts, it's not enough to adversely effect fish.