Dendrob. leaves suddenly losing color and mottling

[RobotPigOverlord]

Hi!
I have had this dendrobium chrysanthum for about 4 months, and until yesterday it had been thriving (new roots, cane growth, new leaves). I was away from home for 1 day, and I was dismayed to find that in the 24 hours of my absence, something terrible appears to have afflicted my plant. New mottling, yellowing, and multiple leaf tips where the color looks like it was literally drained away, leaving the leaf tips to be nearly translucent.

Its not possible that this is due to sunburn, this plant is grown only under a horticultural LED light. My temp/humidity gauge shows the high/low history and there was no heat surge while i was gone. No sign of pests (that i could see). Fungus seems out of the question because there is excellent air movement and the orchid is mounted on cork bark, the leaves just dont have the prolonged moisture on their surface required to support rapid fungal growth.

I only use rainwater (which i filter with a Berkey) to water my orchids. MSU fertilizer + Quantum Orchid once every ~10 days. Kelpmax once per month (started 2 months ago).

I only got into the orchid care hobby about a year ago, so im rather new to all this. It is the first time I am encountering an issue with one of my plants, which happens to be one of my favorites so I am very concerned.

The change in the leaves happened over the course of just 1 day, and did not effect any of the new leaves that have grown in while the plant has been in my care. The plant had a few little black spots that looked to me like sunburn when I got it (theres a picture of the plant I took on June 10th, which shows the black spots), they have not changed at all in the time that i have had this orchid. No new black spots have appeared. The change in the plant is all color loss, mottling, yellowing and withering leaf tips.

Could it be calcium/magnesium or some other nutrient deficiency? If so, what is the best way to ensure my plants are getting enough cal/mag?

What do I do with these damaged leaf tips? Cut them off?

Sorry this post is long, im just very worried and want to provide enough info about the plant's care to help facilitate a diagnosis of the issue.

Any ideas/suggestions/advice would be greatly appreciated! Thanks!