Just got off the phone talking to the company that distributes this.... Yes, it can be used in freshwater aquariums, however do not use if you have any snails - or invertebrates. When I put in the first 10 ml (for 75 gallon aquarium) - white little squiggles formed in the water and my fish ate them! Yikes! I put the remainder of the dose in the power filter mounted on the back of the aquarium ( I run two filters on all of my aquariums). The liquid dissipated in filter so no more (if any) harm done to the fish. The associate from the company actually tested it in their freshwater aquarium - and the same thing happened - but the "Experts" assured her/me that the liquid was not harmful to my fish.
After I give this product a fair shake (a few weeks), I will write a review. The Nitrate level in my 75 gallon was 40 ppm before dosing. I do have some fish that are nitrate sensitive (feather fin cat fish and Congo tetras) so like to keep the level below 40 ppm.
While the advice given at Drs. Foster and Smith is top notch, I recommend you call/contact the company that makes the product first as they usually do extensive testing before putting products out on the market. Note: there is NO warning on the bottle NOT to use it with snails/invertebrates....so I will contact Drs. Foster and Smith today have them update the information on the site.