how you keep track of ppm
 

Oh yeah, one other thing....I just asked about the magnesium. I'm wondering how you keep track of ppm by simply keeping track of how much nutrient solution you put in each gallon? Or does how many tsps. ,1/4tsps ,etc... that you put in each gallon replace ppm if you cant directly monitor it. I've read your horror stories about tds pens.... if you pay that money, it must be a worthy tool, right? If you cant tell, I'm a novice looking for some info, and all 32 pages here have helped me alot....THANKS.

Yes. The directions on your nutrient bottle should say that if you added say one teaspoon of solution per gallon of water will give you a PPM of say 1000 PPM. So if you wanted to keep your solution at 1000 PPM Every time you need to add water to your tank you would put 1 teaspoon of concentrated nutrient solution to every gallon of water you add to your tank.
I just got a flyer in the mail from Aquatic Eco-Systems, Inc. and they have waterproof TDS (PPM) Pens and pH Testers
for $55.50 You can't do much better than that.