That looks a lot like root-rot. You might not keep the plant in standing water, but that soil looks very wet. Watering a Ceph every couple of days is basically the same as keeping it in standing water (unless it's extremely hot.) The root system needs to significantly dry-out regularly. For instance, I water my greenhouse Cephs every couple weeks at most.
It looks pretty late in the game, but I would stop all watering until the soil gets quite dry (contracting from the sides of the pot.) Feeding should be your LAST priority right now, as your plant doesn't look healthy enough to do anything with those nutrients. You need to get your light cycles and watering figured out before you focus on anything else.