Hey there, so I had a plant having a similar sign the rusty, brownish discolored tops when I wasnt overdoing it on nutes. I soon realized it came down to either a calcium deficiency which can show its self in the tips where light is hitting the leaves most; and maintaining a healthy root zone. I believe there are natural versions of calmag supplement such as making a tea or mash out of egg shells, if coarse anything you try do at 1/2 or 1/4 strength and see if any new growth continues to be affected. Some damaged leaves may not fully return to their green glory, but are not useless enough to cut off.
Best of luck.