Please save my violacea seedling! Severe chlorosis

[cavycat]

My blue Phal violacea seedling's leaves have been on the pale side since I got it (8 months ago).

It's been in s/h, under lights, with a heat mat.
Fed rainwater + fertilizer at 125ppm N, +magnesium sulfate

When a new leaf that was just peaking out went yellow and died, I thought it might be a Calcium deficiency and got Calcium nitrate.

I have four other phal seedlings doing fine under same conditions.

The condition was still worsening so I went for the sphag 'n bag method and removed it from the lights, placed on warm window sill (26 C, 78 Fahrenheit). It's roots are in a poor state too. Used some rooting hormone and dilute solution of KeikiPro.
After two days the remaning leaves are almost completely yellow.

What's the problem? Can it be saved?

I'd really appreciate you're help as I'd be devastated to loose this plant.