I just wanted to say... NO aquarium should be placed where a pet can fall in.
I doubt the pirahnas would eat your cat, but my mom used to have a fish tank....
She left on vacation, told a friend to feed her bird, fish, and dog.
The friend didn't do it.
The bird escaped, tried to drink from the aquarium, fell in, and drowned.
The dead bird quickly killed all the fish.
If your brother wants some agressive fish, just set up a african rift lake aquarium.
Still need a large tank (about 90 gallons) and a good bit of filtration... but much more interesting and colorful fish.