Super Algae Destroyer? from Omni

[waterbear]

It is granular trichlor, It's the chlorine that does the killing and the side effect is that it will raise your CYA. The reason it is sometimes used on black algae is because trichlor is very slow dissolving and will release the chlorine to the algae over a period of time when sprinkled right on the black algae spots. You can do the same thing by putting a trichlor tab on a black algae spot.

It is NOT my first choice for killing black algae.