Light: Sunleaves Vitalume 4', T5, 54watt, Just moved back from 12" to 18" because I was thinking they might be too close. On 24hrs.
Your light is way too far away. Set your seedlings about 4 inches away and see how that goes.

Since your water is only 20 PPM, are you using R/O water? Seems a bit low, even for seedlings, but I don't grow in rockwool or oasis.

I have no experience with either R/O water nor your medium, so someone else will have to jump in with the feeding info for your situation.

In the meantime, you can search for feeding info when using R/O water, or just general feeding info for hydro grows. For additional reading, all the info you need is in the Stickies to be found at the top of each forum.

There is also a sticky troubleshooting form that helps us help you diagnose your problem. I would suggest you fill it out and repost.

good luck,