The instructions do say you can dose the entire tank. It just says not effective as spot dosing, which clearly would be tricky on some algae. I've never had a major issue with the other types of algae.Per the instructions...Especially effective for BBA, hair/string/fuzz and most forms of filamentous algae. Ive had fine hair algae which i manually removed, and couldn't imagine how you would go about spot treating it.