I have used epsom salts before. The only possible drawback is that it adds sulfate as well, but as long as you keep up with your water changes it wouldn't pose a problem. Measuring doesn't have to be all that acurate, because it takes a ton to increase mag just a little.
IMO, it's worth testing every now and again just to be sure it's where it needs to be since it's tied so closely to
ca and
alk.
