my friend had this problem in here tank and was caused by bad bactiria in the tank.It can also be caused by aquariums that are not mature inuf for the fish.
I would not add endlers, i would just take it slow and add the about 3/4 fish every 1/2 weeks. I would'nt add a betta it would get frustrated with all the other fish.
If its sinking and not eating it definatly sounds like a swim bladder problem. One of my fish died from bad swim bladder problem. If you can try and feed it some pees that would help.
GOOD LUCK!
I dont think its much to worry about, my red tail did just the same. Its better to give them a cave or something a good size for them to hid in and protect. It worked with mine. :fun:
If i were you i would change to sand because its alot better for corys, plecs, loaches,etc. And i would get the kuli loaches. it just seems a better decission. :good:
If you have a spare one you could use one of the rubber thing that are use to stick filters and heaters to the tank or clean blue tak or something like that. :hyper: