BillD
Aquarium Advice Addict
If you are going to use hydrogen peroxide you have to apply it pretty much directly to the algae. This is usually done with a syringe or turkey baster. From the picture it looks like there are not many plants. Adding phosphate will only make matters worse. You could remove the plants and dip the stalk/leaf portion in a solution of 1 part bleach to 19 parts water for about 2 minutes. This will kill the algae, usually.