Mag floats are great for touching up on algae. Typically they won't get that hard green spot algae, but they can take care of other types. These are for glass aquariums only. Only the large comes with the blade scraper included.
Our personal recommendations for sizing:
Small: up to 20 gallon aquariums - up to 3/16ths thick glass
Medium: Up to 55 Gallon aquariums - up to 3/8th thick glass
Large: Up to 180 Gallon Aquariums - up to 5/8th thick glass(comes with blade scraper)