I would like to know what is the best Roof Protector, to keep the algae from growing back, for the money? I have been using Roof Armor from Cleanertoday.com. But it is $34 per bottle, and quite often I use at least one bottle, and sometimes more than that. I know $34 isnt much, but im trying to lower my costs, while still using quality products...any advice would be helpful!