Well... i used to have a juvie 2'' long... i had him for about three days before he unexpectedly died... water params were all in check, etc. the only thing i can think of is that he must've jumped and banged his head... i will be getting a new one soon... or two... or three.
so my only recomendations for any future arowana keepers... would be to keep the tank sealed tight! and whatever you use to block the tank, don't use something that could potentially harm them should they jump, like egg crate. also you may want to start with a medium sized (at least 5'') aro... since those babies are awefully hard to keep alive
