Thank you Gee! I don't think that the leaves present in this way. The burn is inside the leaf and on the tips of my plant, not on the leaf margins, and the leaf margins are not yellowing first like Potassium def. Maybe I must get some better pics.
Have you ever watered this plant with fish fertilizer? If it was my plant I would root drench it in a tub of water for 30 minutes, drain it to drip-dry, then gently do it again in fresh water. Do this all gently. Let the pot sink on its own, and raise it out slowly.
Then dust the surface with some myco. Not a ton but dust it, then deeply and gently fully water that in with fish fertilizer mixed as per the instructions, so about 1.5ml per litre.
Make sure it's hydrolyzed fish ferts, not emulsion. Emulsion is the really stinky stuff.
Then next watering while it's still quite damp from the drenching, I would water it with blackstrap molasses mixed at 2 tablespoons per gallon, to boost the microbes, then the next watering after that use fish again, to help the new myco establish, and then go back to regular maintenance and waterings.
Most don't realize the importance of feeding myco. It's more important than feeding microbes, but it's easy. Use hydrolysed fish ferts once a week. Myco will be happy forever. The plant will feed it after that.
So to itemize:
Very gentle double root drench
Water to runoff with fish ferts
Next watering molasses
Next watering fish ferts.
She's hurting and none of this will hurt it, but I bet it turns around. It will take a week to stop it, then steady it, then turn it around, so you need to move quick.
If it were my plant that's what I would do.
Heres my logic on why.
It looks like a K deficiency. Too much calcium and/or magnesium can lock out K. So you had a K def, and added CalMag, now it's worsening.
The gentle double drench will remove a lot of loose calcium. Fish ferts are high in K, and everything a plant and more importantly the fungii need. It's a full diet high in good K.
Molasses feeds your microbes while the plant can't produce enough exudates so jumpstart the microbes. Extra myco is just cuz. Its a rescue and harvest won't wait, you need to hurry so use myco too.
Hydrolysed fish ferts, thats important. NOT emulsion. Blackstrap molasses only.