4 plants in and out of good health

So I've got 4 plants, Velvet Bud, just under 2 months old. I have been fighting with them since they were born. I can't seem to keep them healthy try as I might.

I'm running a ~13 gallon DWC system.

Current Relevant Info:
pH: 5.9
ppm: 520
water temp: 68.8F
air temp: 74.1F
humidity: 54%


It looks to me that it could be a magnesium problem but I'm not sure. Should I be feeding plants that large more nutes? Am I having a nute deficiency problem or some other problem? Would really like to think about flowering soon, but I can't begin to think about that if I can't keep em healthy during veg.


Please Help!

Anyone have any thoughts? Please would really like to make this work

No one has any ideas? They are starting to look worse. Planning on doing a res change tonight.

Really would like some input, please folks!