[asylumdown]

At this point I can say with pretty high confidence that hydrogen peroxide is both one of the most useful and safest tools in a reefers toolkit.

It's not completely without risk, but you have to dose a massive amount to harm anything. I use both 3% and 29% depending on what I need it for. Stupidly, I've been storing my 29% in a 3% bottle from shoppers with a home made sharpie label on it. I messed up once and grabbed the wrong bottle and used 45mL of it my tank. A few of my acros browned out for a few days, but that was about it.

If you're spot treating cyano or Dino's with the pumps off, 3% works great. If you need to really clean off a section of more persistent algae, or are looking to broadcast dose to systemically inhibit cyano, 29% (or 30 if you can get it) is probably easier to use. You can also dilute 29% however you want. I'll sometimes make 7% solutions cuz they have a bit more kick when it comes to killing algae, and you can use more of it.

ETA: when I used the 45 mL of 29% by accident it was with the pumps off and about 50 gallons of water drained from the display. I don't think that amount would have caused a reaction of it was full and the return pump had been on