Your plant is being suffocated. Pythium and some salt buildup, it looks like. Rinse the roots under a faucet, do a res change and scrub the res, no nutrients yet just run water and beneficial bacteria. Aquashield or something works well.
hisenberg tea, recipe everywhere, worked for me .but i bet ur temps are too high in water under 70 if u can get there.if your temps are high nothing u do will fix it
ok did you look up hiesenberg tea, this works for me, its all benefical bacteria that will attack the root rot and help with alot of other things its not fert. and to my
knowledge you cant buy it , but posts will explain how to make