Depends on how much H2O2 you use, how much organic material (the gentry call it TOC) there is in the water and how well established your aquarium is.
What is a perfectly harmless dose one day can be lethal the next.
Generally Hydrogen peroxide works quickly and aggressively, it is in fact more aggressive than household bleach.
In a 55-gallon aquarium staying around 40-ml of 3% H2O2 (about 7-ppm H2O2) in a 6-hour period should be conservative. I tend to be a good bit more aggressive, if you are dosing frequently, it is best to be conservative.
Since you have the algae that is a good indication you have elevated total organic carbon (TOC), so in addition to attacking the alga directly you are oxidizing the excess organic carbon, the problem is that once the undesirable organic carbon is gone, the gills of your fish are handy targets.
So the overall answer is to reduce the dissolved organic carbon load, the usual suspects are over feeding and dirty or overloaded filters.