I upgraded my 30 gallon tank with a powder blue jack dempsey (2-3 inches) and Angelfish to a 90 gallon tank a month ago. The angelfish seem happy as always as well as all the other fish in the tank, but the powder blue has changed… he used to follow me around everywhere, and at feeding time he would attack the flakes and gobble them all up in a second.
When I moved him in, he stopped trying to get the food, and has just been staying at around the same spot, near a sword plant. I’ve tried feeding him flakes, pellets, bloodworms, brine shrimp, ect… no good on any of them.
He sometimes goes to swim somewhere else, but most of the time I just see him there. His colors have darkened up by a LOT which contradicts all his other symptoms, but he seems skinnier and much less active.
Even if I try to hand-feed him or put the food closer to him, he just doesn’t seem interested. I haven’t seen him eat in a couple of days. He’s been in the tank for about a month now.
Any suggestions? 
The only thing that I think might make sense is that he feels lost in the tank? Before I guess he was confident because it was his “turf”, but now there’s so much more (bigger) fish than him, I guess he doesn’t feel as confident or something.
Maybe I need to put another powder blue jack to keep him company, but I’m nervous and I consider myself lucky that this jack dempsey turned out to be so peaceful.
I can’t put him in my other tanks, because they all have small fish like neon tetras that wouldn’t have a chance. Right now I would say he’s about 3 1/2 inches.
Ammonia is at 0, nitrite at 0, nitrate at 10, ph at 7.4 — all other fish seem healthy, except the depressed dempsey. :/