It's slow dissolving nature is exactly what makes it useful because it act's as a 'time release' chlorine (as does trichlor in any form). However, it does have drawbacks and should not be used on a continuous basis or if the CYA is high. It is most effective if there are only a few isolated areas of algae that need killing. If the algae is through out the pool it's easier just to 'nuke' the pool and keep the FC level high with bleach.