As a rule, comercial buffers aren't worth their weight in the problems they cause. Used properly with dilligence and an understanding of concentrations and dilutions, they can work, but so can baking soda and crushed coral or aragonite, and without adding PO4 to your tank.
What they mean is that PO4, being a non-carbonate buffer, will boost KH (about 0.5ppm KH for every 1ppm of PO4, if I recall correctly). However, plants use PO4 as a nutrient, and algae are also plants. Yeah, I know, redundant statement. Anyway, what they're saying is that using their product in a tank set up for plant growth will throw your nutrient balance out of whack and possibly cause an algae bloom.
Yes, it is for freshwater tank, although, IMO, these buffers shouldn't be used in any tanks. You say that your water is too acidic, what is your pH? What's your KH? It's very likely that there's nothing wrong with your water except that your lfs thinks it should be different, or some uninformed website or book told you it should be. On the other hand, it is possible to have very acidic water, I know one guy around here who gets pH 5 from his tap.