Excel kills. By law SeaChem can not make that claim in the US on the label unless they spends upwards of a couple of million dollars to obtain FDA certification as an algicide. So Seaschem will not and can not say it kills algae.

They say however that the sole time you should not use it is if you are trying to cultivate algae in any form undluig "Morimo balls" which are just algae. If you're trying to grow Morimo balls do not use this. If you put some in your tank and all your algae dies (which is does, I checked) then you can't blame them - you were told it is specifically not recommended for growing algae.

I know this because Greg, the CEO of SeaChem when it was created, told me around 2002 or so. I probably still have the emails somewhere but I'm sure this policy has not changed at HQ.