Epsom salt, which is magnesium sulfate, is a natural old time remedy. Humans have been using it for as long as I can remember for constipation, and as a soaking salt for general aches & pains.
Some people with ro units will use it when reconstituting to increase hardness, it's cheap & effective. These fish live with it in the water full time. When I use it, I just remove it through regular water changes, it really doesn't harm anything. Folks who use aquarium salt, which is sodium chloride, when fish seem a little "off" should try epsom, it has the same effect in regards to osmotic pressure, along with the benefit of aiding digestion.