I've heard of this as well, but I believe it's not good for the fish, but apparently, that's in dispute.

Here's a lengthy article from ESPN on caring for fish in livewells. If you scan to the very last paragraph, they recommend not using Peroxide because it eats the mucose off the fish - same s if you pour it on a wound on your own skin.

ESPN - Keeping Bass Alive — Livewell

Apparently, some folks do it though with good results. I see several google links that appear to endorse it from posts on other forums. Personally, I'd pass on it.

Wannabe...