Dartfish help please

Okay...so a few months ago I bought 3 scissortail darts from Bluezoo. The first one was a DOA...the other 2 seemed to do great...for a while. I quarantined the 2 for 12 weeks in a 10 gallon! They lived in a half moon shaped PVC pipe together. I noticed after around week 6 one appeared to be more dominant and I guess you could say would go after the food first and the other one would stay in behind him. I spot feeded him so luckily enough he was able to get food. Well after 12 weeks I put them in my 90 gallon. Lots of LR and they are the ONLY fish in there. Well at first they were both out and looked to be having a good time...they werent out all the time but you know here and there. They would both especially come out about 10 mins after feeding. Well eventually the one would stop coming out and the other started to come more and more out. I would try and spot feed the "weaker" one and the more dominant one from what I could tell would dart back into the den and maybe...maybe rough him up. I just would see him go in there and see a lot of movement which lead me to believe he was being bossy. So with time it seemed like the one never even moved...it seemed to always just lay in one place. Now as of the past day I noticed it has moved a few large rocks down the tank and is in a new area. The one dartfish does not bother it...I offered the weak one pellets and the other one did not come near. I feel the dominant one realizes he is on his way to dying and hey maybe thats enough for him not to worry. The weak dartfish now has a red line where you can see his gill...mainly from malnutrition I believe and some what heavy/rapid breathing...(signs of a soon death) and its not really moving much. Anyways...this has been an on going thing for about 5 months now. Any ideas? Maybe a disease...I feel it was more of a stress related issue...the fish also has a bowed appearance as well. I will also note that when at the time they would both come out I would put food in front of the weak on and he wanted nothing do do with it. Even in the quarantine tank he seemed to have a loss of appetite towards the of its quarantine procedure.