Never heard of any ill effects from normal usage. Just read up thoroughly and stay on top of what you are doing. There are really bad things that can happen if you do it wrong.
Make sure you have a good skimmer, rated to preferably twice the capacity of your tank to provide good aeration. The bacteria produced will lower the oxygen content in the water. Overdosing can/will produce a bacterial bloom and likely suffocate everything in the tank.
Do your research and take your time! Nitrates/phosphates won't drop overnight.
Good luck, it's been working great for me so far.