Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
All chemical algae treatments are potentially harmful to fish.
As a real-world example, have you ever tried using H2O2 as a mouthwash? Use a little, preferably mixed with some water, and it leaves your mouth clean and fresh. But use too much, for too long, and it will leave your mouth irritated. More still will seriously burn you.
It's trickier with fish, since their gills are more sensitive than your mouth. And the line between being effective on algae, and causing serious injury or death to fish, is smaller.
The "One-Two Punch" uses several techniques - like optimizing flow, and relying on a combo rather than H2O2 alone - to reduce the risk of H2O2 use. However, it does not fully eliminate risk. Plus the full treatment and dosages as I've written are intended to be a strong one, meant for serious situations where you are willing to take some risk in order to get the desired results.
You can of course modify the treatment, and therefore the risks, to suit your situation - and I encourage people to do so. If the fish can be removed, then AlanLe's suggestion is a great one.
If you only have a small bit of algae, then there is no need for my treatment, as there are others which will usually suffice. H2O2 spot treatments at no more than 1 tbsp. per 10G are virtually risk-free, for example; as once that amount of H2O2 disperses throughout the tank, it's too diluted to have any real effect on fish or algae.
I hope this has adequately answered your question. You are welcome to post in my thread or this one as you prefer with any questions or concerns, I will answer them all truthfully with no bias to my own method. And I welcome all feedback, positive or negative.