after reading all your post in and out and upside down i have come to the conclusion of your problem which is stress induced by the other fish.. normally the most abused cichla becomes the whipping boy which usually wont kill him or her but enough to stress them out. ( this happens usually with ever group of cichla) now for the reason why your tem isnt eatting normally is most likely between the stress and the nitrates.. normally they will have no problem being the whipping boy ( you will occasionally have to make sure they enough food to keep up with the growth of others) sometimes they will even have the brightest colors out of all your cichla. so in conclusion it seems to me that the mix between the stress and nitrates took a negative effect on your tem.. he should be better after the water change or tank move whatever you do or have help.... wish i lived closer, i would swing by with my python and take care of your tank.. i hope the info helps and i also how you feel better