MgSO4.5H2O or Epsom salt, as you can see, doesn't have any Nitrogen in it. Meaning there's no correlation with any compound that has nitrogen in it.
Consider it as adding salt to the tank.
Is it possible that your ammonia was already high and you just happened to test for ammonia after adding Epsom salt? Or did you do a before and after test?
Do you have sponge filter/canister filter etc. running in the tank which provides biological filtration? Most probably it is just lack of biological filtration.