ya a red bellied pacu is not a good choice. I got one and put it in a 10G when I was a newbie back in the day and it grew sooooo big in less than 2 months... poor thing used to smack into the wall and jump and hit the hood, he also up-rooted plants like a maniac... I also was feeding him wrong, they like to have a lot of vegetable matter. I gave him to a friend and he killed him on accident
for your 30G why dont you do a community or a small aggressive chiclid tank?