I don't think it's necessary to get scientific about it (but I may be wrong. I often am!) I would think maybe 'a splash' or 'a little' would do it. Of course you could wrap your reservoirs or use dark-colored ones to shut out the light so you won't get the algae in the first place. I just run mine through the dish-washer every time I see the algae showing up--algae doesn't hurt the plants, except of course to plug the wicks if it gets too bad, and I always worry a teeny little bit that too much algicide might.