It's not nute burn. Looks like Mg deficiency...... add expsom salts or cal/mag. Are you fertilizing? They appear too light green too.
So:
1. Water is not as important as quality of water. What's you pH and ppm?
2. Fertilize with a high quality hydroponic fertilizer (a complete fertilizer).
These should be your first 2 steps.........
FYI., 'fertilizing', 'feeding', 'giving nuts or nutes' (nutrients) pretty much means giving 'other than plain water'.
Your PH is a shade low in your water, not bad, but towards the lower end of the range where you want it. (6.2-6.9 is about the max range). I wouldn't worry about that too much right now, unless your soil PH is low also.
Like SSHZ said, 1 tsp/gal of regular epsom salt should help with what looks like MAG deficiency.
Purple stems is a phosphorus (P of the NPK) deficiency.......so it appears right now you are deficient of P, N, and MG. Other problems will follow shortly. You'll need to find a complete fertilizer and start fertilizing pronto. Return the pH meter to Home Depot- it's inaccurate for your situation and a waste of money. There is a ton posted out there on what to do, what to buy and how to go about growing. You need to spend some time searching out a good spot for beginners and read up on things. 6.3 pH is perfect for plain or fertilized water going into your pots.
1. Did you add lime to your soil when you started- that would have helped with the Mg shortage.
2. Usually in the 2nd or 3rd week of growing you'll need to start fertilizing.
3. You'll need a good hydroponic pH/ppm meter to tell you your pH and fertilizer strength. This is the most important purchase you'll ever make!- buy a good one.
Go to www.hydrowholesale.com and request a catalog. You can learn a lot by reading that 172 page catalog. Then shop around for the best prices on the things you need.
First off.. everyone just chill out.. calm down and breathe..
RS, The damage that was done is done and will not heal. The leaves that are yellow will continue to yellow and fall off. This is expected for the state your plants were in. Yellow leaves don't go green again.
So, check out the new growth.. What does that look like? It looks better to me. It needs to grow out of the damage. You're not burning your plants. Nute burn looks completely different. You'd have dark dark green leaves with all the tips burnt.
Don't panic FEED THEM and expect all those damaged leaves to come off at some point. If you don't feed them you can't expect them to grow.