Before you add salt--who else is in the tank? Generally, I keep 1 Tablespoon per 10 gals in my FW
tanks. Mollies would like more, but I have not figured out how much they need. I have mine in what will be a BW tank, but I want to get everyone in there before I change from FW
Food can cause a swollen abdomen--either over feeding, or the fish may develop a blockage. If it is a blockage--try blanched peas (you will have to peel them). How often do you feed your fish?
I use those test kits also. I was wondering if you were using strips and getting "off" readings.