From what I have read it attacks the lower forms of algae first. The company doesn't think it will harm macro algae in a refugium. However, some people say it has weakened their cheato. I would be careful when using the higher dosage i.e. twice per week. Haven't read anything when using the maintained dosage i.e. every 2 weeks. I am considering dosing my second aquarium which only has the film on the glass and gets little round green dots that takes some time to remove. It's an acrylic tank so I can't use a razor blade and the plastic doesn't do the job. I don't know if the maintenance dose will work, but for the amount used, 1ml per 10 gal., it is inexpensive. For my 200 gal system this only 40 ml per month so an 8 oz. boltle would last about 6 months. Cheap to get a prstine aquarium, and less work. I hardly touch the aquarium l'm dosing.