I'm looking at my male Syn right now--he's in a 540 gallon tank with plenty of room, but he's swimming around with his fins all flared up, shaking his head at my Blackbelt everytime he swims by, looking outside of the tank at me to see if I"ll get up and drop some more krill in "his" tank. Any fish in that tank that object to him behaving that way--they pay the price.
6 months ago, he was the wimp of the tank, I almost re-homed him because I thought he just wasn't gonna make it in that tank.
Totally different fish now--that's what happens when they go from juvenile to adult fish. OP's Syn is 9 inches--it could very well go through the very same metamorphosis when it hits 10,11, or 12 inches.
But OP's Syn, if it changes like mine did, won't be in a big tank like mine. And it won't be in a tank with some Tetras, Danios, Convicts/HRP's like yours. Nah--OP's Syn will be in a cramped tank with a Flowerhorn, an Oscar, and probably a new Jack Dempsey.
And if the Syn doesn't go through this change, I can assure you all that Flowerhorn will.
And then--if OP has a problem and tries to put a divider in that tank--that's gonna make it worse instead of better.
So, you'd have to put 2-3 dividers in a 150 gallon tank?
Instead of looking to buy new fish, I'd be looking to buy a new tank.