My understanding was that PMDD (yes, I got this from you Richmond, thanks)
consists of "macronutrients" and you use it along with a "trace mix" which I
thought was, for example, TMG. I guess I was wrong.

PMDD has trace mix as well as other stuff like nitrate.

From reading on Thekrib, Potassium, Magnesium, and Nitrogen are important.
These are the chemicals in the PMDD I bought from Richmond. Are they also
in TMG? I know it says "no nitrates".

TMG doesn't have nitrogen (nitrate). But a lot of people don't need it.
Nitrate needs to be added to high-light tanks, or tanks without many fish.
Otherwise, fish provide more than enough of it. In addition, tapwater may have
quite a bit of nitrate in it, depending on where you live.

Since your tank is a moderate-light tank (110 watts over 55 gallons), you may
not even need to add nitrate. Get a test kit and measure first.

If the bottom line is, just use the TMG, that simplifies things. I have a
1/2 liter bottle.

I would use just TMG, at least at first. Later on, you may find you need to
add nitrate, phosphate, or other stuff. (But you might not. In a
moderate-light tank, TMG and regular water changes are often enough. That's
all I use in my moderate-light tank.)


Leigh

http://www.fortunecity.com/lavender/halloween/881/