fish chasing

I had a male betta fish and 2 neon tetras in a 5-gallon aquarium. The aquarium cracked, so I drove to the pet store and got a 3.5 gallon. My betta never bothered the neon tetras before but now he is chasing them. What should I do?

Just a few days ago, I bought a powder blue dwarf gourami and a Columbian tetra. I am keeping just the two of them in a 10-gallon tank. When the dwarf gourami isn't hiding in the structures or in the corner, he's sneaking up on the Columbian tetra. He isn't fighting his tail, but he is poking...