It's interesting, but I am willing to bet the technology is cost prohibitive for fishkeeping applications and thus we haven't heard about it since. Either that, or it was a crock.
actually, wild-type guppies and sailfin mollies(traditionally freshwater fish) can be kept in full strength saltwater and most marine fish can be aclimated down to brackish salinity levels as low as about 1.012. so technically you could keep freshwater and saltwater fish together. i had a knight goby and a yellow tail blue damsel in the same tank together for quite a while(althought the knight goby is a brackish fish)
They had this stuff a few years ago at my lfs except it was called "magic ocean". The LFS had a display tank using it that had a yellow tang and some swordtails in it. I decided to buy some and test it for myself so i setup a 10 gallon tank using it. I opened the bad and discovered that it was just some fine coral sand and some salt mixed together! You wanna know how it works???
IT ONLY RAISES THE SALINITY TO 1.010!!!!
Not high enough to instantly kill guppies and swordtails but not low enough to kill the saltwater fish...