Depends on the type of algae. Treating the whole tank with peroxide isnt an option, its for spot treament on decorations or plant leaves ect.
Fill a dropper with a small amount, turn your filters off, put you hand inside the tank and aim the tip of the dropper at the algae, wait for the water movement caused by putting your hand into the tank to stop and slowly squirt it.
Have you thought about less light and more water changes?
Fish die belly upward and rise to the surface. It's their way of falling.