over feeding can cause the roots to close all the access to the soils nites and will cannibalise the leaves as you can see beginning now. they use the green , stored chlorophyll to stay alive while the roots search for a less harsh source. . They need a basic reset of the medium by flushing out the excess nutrients 1st. a simple process with lots of info online too. It can be rescued with a little time and care now. Good luck bud