No answer from me, but I hope someone has comments. I had a Warneckii (similar) and for several years I've tried changing pots and care, only to keep having brown edges return time after time. I am on a well, so do not have fluoride, and usually use rainwater anyway. I have finally put it out of its misery, deciding it must have a systemic infection.
for yours though!
You might be asked to post pictures, if possible. Of the whole plant including pot. How long have you had it? What kind of light exposure?