Beyond the mineral benefits, it also is fully of negatively charged ions, and works much like the "ionic breeze" air purifier by attracting and trapping toxins. It also polishes water. A lot of people see my water and ask how I get it so clear, this is part of it.
It's true, the dosage in a fish tank, would be very minute. The product I sell has a suggested dosage of .12cc of powder per 10 gallons! At this dosage, it will raise the GH of the water minimally, and as far as I've seen, it's a temporary increase. By minimally, I mean around or less than one GH point.
As far as it melting your plants, I have no idea what could have caused that. I've never had a problem like that, and I quite a few extremely sensitive soft water plants. Although the reason I've never had that problem could be because I've always used a clay supplement, therefore the plants haven't had to get used to it. (although I rarely have a problem with newly purchased plants either)
Hope that helps!