I'll add experiences on all atf and payara since more or less applies.
summary of death by tank:
9x3x3 fully planted and decorated
current size range 5-12" fish various atf, payara, cudas
2 juvi GATF 3 inch or so - both assumed to have been eaten, or shanked to death and bodies consumed by scavengers as no bodies were found.
2 5" vatf and 1 8" batf - same as above
1 6" vatf - tank jumper
2 juvi raph - just didn't make it, died in seperator
4 6-8" raph - shanked to death or bashed itself into starvation mode
2 4" scombs - dissapeared with out trace. Most likely shanked to death and eaten by scavengers
12 6-8" tats - after months of growing from 4", over a period of few months either by sudden light on/off or fear of tank mates, bashed themselves to death. ultra skiddish.
8x2x3 sand substrate only
large channa micro, pbass, arrowanas
16" vatf - consumed by channa after months of cohab
4x2x3
rock substrate, and rock decor
2 4 inch brevis - one consumed by lates C., other by bagarius yarelli
So if we net out the deaths by bad tank mate choice and juvi deaths by not caused by tank maktes or poor water conditions, the general cause of death is shanking by larger tankmates. VATF are the main shankers. But "shanking" isn't the most accurate description, it's more like slicing.
The survivors are either exceptional for example the lone raph now 12inches. it's been sliced and diced but recovers. with payara, the important thing is their psyche. Once a payara gets scared enough not to eat, the battle is uphill. If they eat, they heal just like any other.
the vatf are the most aggressive, and from what I can tell the ones that do most of the damage and skirmishes. other atf and payaras are not far behind. This means injuries are inevitable. they are incredible healers. So then it's just a matter of how bad the injury is, which is highly correlated with the size of the fish getting attacked and the size of the attacker. If the proportions are within healing parameters, likely victims will survive. Most of the fish i lost were the smaller ones or the less aggressive.
if a fish acts different, shys away, doesn't eat, then it's likely it wont live. Some other things I do are to keep the tank stalked with live feeders more or less 24-7. they act as dithers and to keep the fish full. full stomachs = less aggression. Plentiful good supply, i infer less territorial instincts. I also keep water exceptionally clean and keep a constant salt content. this helps immensely with healing.
payara i'd say require a larger tank like for like to an atf. they need the space to make an escape. an atf darts around and ends up in different places. payara do a loop and end up in same place. space required for a loop is much larger than a linear spurt. With out the room to escape, fish tend to be more nervous, whcih contributes to death.
I've attached photos of a recent injury to one of the GATF, smallest guy in group. fish is eating, swimming with the pack and should heal up fine, as have seen numerous injuries like this recover. The photo of the batf, new add to tank, got tore up scales and puncture in gill plate. Also one of the smaller fish in tank at around 7". This fish unlike aforementioned is shying away, not eating. Could be do to newness to tank. In either case wouldn't be suprised he doesn't make it.
Armatus can be kept in groups with out much aggression. Just need space with decor to mark territories. I suspect denser is better.
current flow is also very important for these fish. I use a Koralia current turbine for this. keeps the fish busy, which decreases injury one another.
