That unfortunately is not true. I was born with a Mg deficiency (hypomagnesemia) and was originally prescribed Epsom Salts from my doctor. I HATED IT! It was always bitter and never tasted good. I refused to drink it while I was little, and fortunately for me I was able to find the tablet form when I was 8. Granted, I couldn't (and still can't) get by with 1/8 tsp in 8oz of water.
I will tell you that most Americans do suffer from lowered Mg levels because most of us eat like crap, so it would not surprise me if your level was a little low.
The best way to determine if you have a deficiency is to have your MD perform a fasting blood test.