Wow.

OK, I'm going to sum up some GREAT advice given above:

TAKE A ZIPLOCK BAG OF WATER, NO AIR, TO YOUR LFS AND HAVE THEM TEST IT. This could be as simple as a poorly calibrated refractometer. I can't tell you how many thousands of fish a refractometer has killed. In any case have them do a complete test of your water.

That may NOT get everything. Something as simple as a penny in the water, stray current, soap, a beer, etc. could also play a role.