are you using tap water??
If your testing tap water straight out of the tap, do not mix the saltmix, Let the water sit for a few hours and test then. Readings will be different. make sure you using a freshwater test kit.
If your water is not the cause then its either over feeding which will promote major nitrate issues as well, which are harmful to fish, or it could be your saltmix itself. Most these days are phosphate and nitrate free, but i do believe there are a few still out there that are not like this. not 100% sure on this though.
another thing to watch is your filtration, never cleaning the filter on a SW tank will causing huge increases in nitrate and phosphate within hours.