Sick convict?

I picked up a used tank and fish yesterday. I'm going to be using the tank for mbuna, but the tank came with a 4" convict who seems to be out of sorts. He hangs out in the bottom of the tank and always seems to be pointing down at the gravel. Is he sick or is there something else wrong? Swim bladder?

The tank is established with good water parameters. All other fish seem to be doing well.

He was in a 55 with 2 4" silver dollars. His new home is more crowded in a 29g with 3 4" silver dollars, 4" angelfish, 2" cory, 6" pleco. It has new, treated water, but both filters are established (Penguin 170, AC70).

The pleco was sucking on him this morning and I found one of my silver dollars stiff at the bottom of the tank. Both were alive, but barely. I put them in the freezer a few minutes ago after doing a 10g water change. The previous owner said that the silver dollars were 9 years old. The convict was 3, I believe.