Aquariums: Tiger Barb Trouble

So I’m having a bit of a unique situation right now.

I adopted a hyper aggressive Green Tiger Barb that was surrendered to the pet shop for being so aggressive he was bullying the crap out of both his own kind and other fish in his community tank. I personally witnessed him harassing the clown loaches he was with to no end. They let me take him because I basically promised he‘d have a massive paradise to himself.

I now have him in a 40 gallon by himself with 6 neon tetras. While he has left the neon tetras alone, I worry because his swimming is fast and sporadic, and he seems super stressed. I’ve never kept Tiger Barbs before, so I know they’re supposed to be active, I don’t think they’re supposed to be this active.

I‘m not sure if the solution is to get more Tiger Barbs or let him do his thing. On one hand, the tank is super big and if he was mean, any other fish could simply flee and hide in the overgrown plants. On the other, he is on the bigger side now, so he’d be twice as big as any new fish I’d get for him could be, thus dealing twice the damage. I don’t want to introduce any new fish only for them to meet their doom. Plus, with his over-the-top aggression, I worry it might just stress him out more.

I’m really not sure what to do here and feel stuck between a rock and a hard place. Any advice would be appreciated, thanks.