Pirahna are not a great idea unless your new tank is going to be HUGH - 150G +. Ditto for Red Bellied Pacu.
You can go to http://www.cichlid-forum.com
to get info about different types of cichlids - again, most require a larger tank and there are compatibility issues, so you really need to do your research first. Lots of great info on that site.
35G barebottom: 2 boesemani rainbowfish, 4 congo tetras, Low light plants (1.5wpg) attached to or planted in my own handmade ceramics - Anubia v Nana, Anubia v Barteri, Red Rubin Sword
2.5G - Spot (beautiful betta - Soft pink with red spots on his fins. Java Moss.