Oxygen
Hey, Can I ask you how much aeration you had in the tank? Are there airstones in addition to the filters? I am almost positive that the water movement was not a factor, but if it was a silver arowana or australian than it could very well have been from over-oxygenated water. There are a lot of cases of such deaths, and I have witnessed a few myself. Ph=8 should be alright, not enough to kill the fish, and it doesnt sound like the ammonia was at a critical level, at least not high enough to be sure. On the choking issue, what were you feeding it? 4-5 inches shouldnt have had a problem with feeders . Seriously, I would consider the oxygen hypothesis before anything else.