Fungus Gnats or pH?

Well, the leaves are curling under at the leaf margins and 1 branch fan has started to brown and die off! At first I thought this could be caused by my tap water (don't use it ne more) being very high in pH like around 8.2. But I also have a slight (hopefully) fungus gnat problem. Here are some pics of the damage.
I ordered some "double death" predatory nematodes to kill the gnat larvae, and have switched to RO distilled water. Anything else I can do?

The gnats (larva) munch on the roots; the gnats don't eat the plant. But even then not to any degree you need to worry about.

The leaves curling under, if in flowering, *can* be a precursor to a mag def.
If you feel it might be a pH issue, check the chart for mag availability at your pH.