I've never tested water at different salinities.
At 15 ppt it should work, as long as you already have the units, why not not try, although my guess is, it may not react as immediately (especially when the tank is newly set up) as 35ppt sea water does.
I have tried a number of the smaller "commercial" fractionation units in fresh water, and none worked to my satisfaction.
My DIY 4 ft tall cascade units always worked much better, and foamed more consistently. But....
There seems to be a threshold of the concentration of dissolved organic compounds, where under a certain point (as in a newly set up tank), foaming ceases, maybe because I'm anal about water changes and below 5ppm nitrate, my 4 ft tall unit didn't always produce oodles of foam suggesting, it varied.... if water is too pristine....fractionation won't fractionate in it. Usually right after a water change, foaming ceased temporarily
I have seen serious koi pond keepers build 10 to 20 ft tall cascading units to provide constant foaming for large ponds.
Most waterfalls in nature always produce foam. There always seems to be billows of it under Niagara.
The Cedar creek near where I lived in Wisconsin, produced those large billows of cloudy foam in spring, but not most of the rest of the year, and Lake Michigan, right after storms, maybe because of stirred up nutrients and heavy wave action had foam, but most of the time, its average 0.02 ppm nitrate level of the lake water seemed much too nutrient free (at the level my samples were recovered), well below fraction threshold.
In my job as water chemist, I measured nitrate in Lake Michigan water daily from samples recovered from 50 ft below the surface, 50 ft above the substrate, and 1 mile from shore, because this was where the drinking water intake was located.