I was driving but now home. To touch up on this, there has been an influx of these fish are coming fromn Sri Lanka and it is suspected that a method of capture is contributing to this trend in which the fish appears healthy the first few days and then weakens quickly with loss of appetite and strength and as you found laying on its side, thin body and breathing heavy until death. It may be due to cyanide capture but unconfirmed.
Sorry to hear of this and being the bearer of bad news. Often in this state of health , recovery is very low as they are moribund and adding air stone may or may not help, but there is no additive that can be recommended for this