MgSO4 + 7H20 is Magnesium Sulfate (Epsom Salt) a hydrated form of MgSO4 a Magnesium Sulfate salt (in an anhydrous form) ...
Most aquatic literature use all of the above interchangeably ... and in general they are referring to Epsom Salt ...
If pure Magnesium Sulfate is exposed to the air, it quickly becomes hydrated. So an easy way to check to see if a commercial product is in an anydrous form, is to see if it shipped with a silca gel pack to absorb moisture from any air that it might come into contact with ... no silica pack, and its probably Epsom Salt ...
And if you actually did have an anhydrous form of Magnesium Sulfate, it wouldn't hurt anything to use it in a PMDD recipe ...
P.S. There are a variety of naturally occuring Magnesium Sulfates, some stable, some not:
epsomite MgSO4·7H2O, (stable)
hexahydrite MgSO4·6H2O (stable)
pentahydrite - MgSO4·5H2O
starkeyite - MgSO4·4H2O
sanderite - MgSO4·2H2O
kieserite MgSO4·H2O (stable)
You will find a lot of Mars Rover space mission fans expressing great interest with regards to various hydrated forms of magnesium sulfates and other minerals on Mars ... because the hydrated forms are often picked up on by fans as proof of the feasibility of life on Mars !!!
I wonder what a Mars Aquascape Biotype would look like ... anyone up to enter one in the AGA competition?