Need help for first grow

So I have two 2 1/2 + week old plants growing in a DWC system, with two air stones under neath each net pot being pumped air into by a 60 gal pump into a 40 gal tank with 21 gallons of water. Right now the water has a ppm of 250 and im using BC GROW and BC BOOST, and the ph in the tank is around 5-6 (although it hard to tell considering that im using ph papers). Im also using a 400 watt MH conversion bulb about 20 inches away to grow them.

The problem is with the plant on the right in the photos, the leaves are droopy and I haven't been over watering her, especially now with how sick it looks. The plant on the left in the same tank has shown non of the same symptoms.

Any advice would be really appreciated its my first grow and I don't know what going on with my plant.

BTW the plant on the right that looks sick is a couple of days up to a week older then the plant on the left (the healthy one) .

Here are some photos I hope someone could help, it seems to be getting worse and I really dont know what to do at this point.

how do the roots look,also are they the same strain,its difficult to grow differant strains in dwc or any hydro system that several plants share the same res

Are they in the same solution? If so i dont think its a nutrient problem. Do you have a fan? How is air circulating ? Whats your temperature in your room. See if the roots are getting enough water

question about your system. Did you build it yourself? Are the roots long enough to get in the water, because if not then you should have some type of small water pump pumping the water up to the roots until they grow long enough. Go buy a cheap humidity/temp gauge at Walmart for about $4 and set it in between your plants. I'm still newer than most, but when my Bubbleponic bucket clogged then my plant looked just like that till I realized the roots weren't getting any solution. Just my opinion, but something to consider.

Well these are just some seeds I had lying around that I found out of my years of burning, but I assume there both different strains. I also purchased some Vanilla Kush and LSD Barney's, but I thought I'd see how these plants turn out, and test my hand out at growing.

Also the net pot is exactly 3/8 of an inch into the solution and the roots are in the water and have more then a sq ft of space each

The roots look very nice and white and fish bone looking like. And yes I have a fan running on them 24/7 with and air conditioned room keeping the temp at around 65-75.

So any ideas on what I should do? Thx for the responses
And I aslo did build it myself, should I maybe raise the water level? Or just cancel the grow start again with the same strains?