PMD drops and heaters

Kevin: I'm interested in the Poor Man's Dupla Drops:

The current PMDD formula is:

  1 Tbsp Chelated Trace Element Mix (7% Fe)
  2 Tsp K2SO4
  1 Tsp KNO3
  2.5 Tbsp MgSO4.7H2O
  300ml distilled H2O
  0.5ml 9M HCl

But can you please explain what the letters and numbers stand for and where
one can get them? (I know what the H2O is.) Would one go to a hydroponics
store to find these nutrients?

Also, someone asked about heaters. Ebo jaegar (sp?) and Visitherm. I have
them both. The Ebo was more expensive and in my opinion much harder to read
and set. It also got moisture inside it after a couple of years. I asked at
the store about that (it continued to work) and the guy there said that it
happened to all of theirs but didn't seem to matter. Putting my hands in
the water with a heater on that had moisture in it really unsettled me so I
bought another Visitherm. (I had one in another tank.) The short of it is
that I like the Visitherm better and have had no problems with them.