BTW: you don't use saltwater to top off either. You use regular water as you're just replacing the evaporated water and salt doesn't evaporate. Don't discount the fact that if you are topping off w/ saltwater your SG
is probably bouncing all over the place as well. Inverts don't particularly care for that either.
But if you did a 40% change w/o conditioner.. Probably a good bet that the chlorine/chloramines whacked your inverts. Surprised the fish don't have signs (gill burn, swollen eyes, etc)..
Not good, not good at all...
If you don't have a test kit, do you have a refractometer? How are you testing your PWC SG