1. What is a " 12 plant, 17g DWC system." Are you saying it weighs 17 grams? If so, are you saying the whole system weighs 17 grams, or just each plant? I got to assume the plant, but not sure how that would matter, or how you weighed it.

2. I'm not familiar with the nutrients you state your using. But my first thought is why are you trying to alter it's composition by adding the Epsom salt in the first place? Does that hydroponic fertilizer not contain magnesium? Is it not for hydroponically grown plants? Or are you just thinking adding more of whatever you can couldn't hurt?

3. Are you asking what pH your nutrient solution should be?

4. I'm not sure how a combo EC/TDS/pH tester would help answer any of that. Other than what the pH is, but wont answer what it should be (much less how to mix your nutrients, beyond the overall strength).

5. Have you contacted the nutrient manufacture to ask them what they would suggest in how to mix them?

I suspect he meant 17 gallons DWC system, but I too was unsure.
And I agree, contact the manufacturer for their nutrient recommendations.
Until you're ready to do some figuring on your own, it's best to use what they recommend.

1.) 12 plant, 17 gallons.

2.) Maxsea is a water soluble fert, I've seen it used from everything from Orchids to roses to trees, and a few sites said it was okay to use for a hydro system. It doesn't contain magnesium, hence the Epsom Salt.

3.) I assumed that if anyone was using Maxsea in a hydro system, they could tell me where I'm at with the nutrient solution, too strong, too weak, etc. I can't measure EC, and all the information I have at the moment about my water is the pH of the plain tap.

4.) Lettuce does best at an EC of 1200 microS cm-1, and a pH of 5.8, based on Cornell's handbook. They're using Pete's Hydrosol, so the actual breakdown of the nutrients are a bit different between the two nuts.

5.) That was my next step, though I'm probably going to switch over to a more applicable nutrient soon.