In your 150g you could do an acanthicus adonis or acanthicus hystrix for awhile, but you'd need to upgrade or move it out after a few years. They get big fast and can put up with aggressive fish. There is always a chance of incompatibility with the bichir.
A fully grown L200 can live in a 150 for life and they are plenty tough, active all day and not terribly pugnacious. Just defensive. And they get to between 6" and 8", which equals a LOT of poop for a 150 and is probably a most manageable size range.