Standard dosing Excel doesn't harm snails or shrimp. Double dosing, personally, I have never tried. But I don't think it would do much harm if any.
But, if the algae is not all over everything, targeting the areas where it is can also work. Suck some into a syringe or baster, squirt a bit right on the clump of algae. Make sure no critters are close by where you aim it.
I've heard of this being done with hydrogen peroxide too.. squirting it right on algae covered areas. It quickly converts to hydrogen and oxygen, so it does not harm most animals, though at high strength it's a powerful oxidizer, which is what Excel and Bleach are as well. You can get hydrogen peroxide at various strengths, the drug store kind is usually 3%, and the kind used to bleach hair is 6%. Also called 10 volume and 20 volume. Food grade HP is also available at, I think it's 25 or 35%.. and that usually is at health food stores. Used to disinfect seeds and sprouted seeds. Can be diluted as needed.
Best to dilute a double dose of Excel in some water and then add to the tank, water and all. Excel lasts about 24 hours or less before it's gone.
