I recently experimented with H2O2 as a supplemental treatment for Cryptocarion irritans. I was feeding a medicated food due to the reef environment and wanted to try and irradicate the free-swimming stage, as the infected fish tended to become re-infected within 24 hours or so. I used 30% H2O2, which is very strong, 10x what one typically purchases for hair or disinfectant, at a rate of 1ml pro 100l (approx) into the current. The results were interesting. The water became very clear as it oxidized all of the organics it came into contact with, but the effect is very short term. I found I must dose 2x daily to control the parasites and then not 100%. It did help, however.
Of the other inhabitants the only stress factor was observed in a Heteractis magnifica, which expanded very quickly after treatment, but recovered inside of an hour. It only did this once. Could be coincidence. There are various Acroporas, Montiporas, Favids, disc anemonies, shrimps, sponges, tunicates, serpent stars, etc. A typical reef. None showed stress.
As I run a biological filter system (zeolith), I was concerned that I may damage it. Certainly, H2O2 is not particularly positive for any micro organism, but I couldn't note any change in the effect of the filter. Actually, it may have benefited as the PO4 has started dropping, again, two weeks after the treatment. It never rose, but it was stable ar 0.14. It is now under 0.09 and dropping. This is merely annecdotal, but the H2O2 may have favoured the aerobic bacteria or the macro algae.
In any case, at the dosage I used, I found it safe and effective, but not a magic bullet!