Oh, I thought you wanted to clean in between water changes.
In aquariums, you simply drop the water level to expose the algae and spray on some peroxide to kill it.
You should kill most stuff in the water if you put about 5ppm ammonia in, that is enough to even kill off nitrifying bacteria.
3% of 25 is 3/4, you can get cheap peroxide in large volumes at the pool store. Great for zapping a black pool back to blue.
I run drain to waste, but if I were to do a reservoir. I would add a simple aquarium sponge filter or one of those box type filter that use cotton wool like floss. This would keep the reservoir oxygenated and round op some of the excess ammonia. Algae can not exist without ammonia as a food source. The spore has no mechanism to absorb nitrates other than eating ammonia.