I've been keeping Mono sabe's a Long time!
for some reason a round-ish tank like a 35 gallon hexagon works best, it seems like if they can swim in a circle they don't bully each other quite so bad.
l kept five Mono Sabe's in the 35 gal. hex.
I can tell you they do best swimming in a circle where they kind of school together and get lost among each other rather than singled out and picked on.
I now have a 54 gallon bowfront aquarium,(you'd think bigger is better?) I started out with 9 quater sized Mono's.
7 of them made it to maturity (about the size of an adults open hand) one by one they have been bullied to death. leaving only one remaining and he hides all the time.
These are beautifull fish but a lot of headaches!
they seem to get along well when young, but as they mature they get VERY aggressive!
Every "Expert" has told me to keep them in odd numbers, (they swear they do best in odds)
Another thing I have been told is to have other fish that are fast like the Mono's to distract the "pecking order" they claim it kind of breaks up the fights.
This worked well for me with Puffer fish, but they're such finn nippers that I returned them.
(Who wants Mono's with swiss cheese tails?)

Hope this helps some.
Kipper

If you happen to email me, title it "Mono's" so I don't toss it as spam.

I have some links to other brackish water fish as well.