There are four in there. I've had no issues with aggression at all. I've kept it heavily planted, with plenty of breaks in sight lines. I think you could do ten in a 40, no problem.
Hello all. I've been fish keeping for about a year now, and have gotten really obsessed. There's a 20 gallon pea puffer tank in my office, and a 55 gallon community in the living room. I'm here to learn and share ideas. Cheers!