Betta Fish
Also known as Siamese fighting fish, the variegated variety behaves like a lion in a pride: “One male gets all the females,” says Bernie. Don’t introduce another male, or you’ll spark some Thunderdome action as each nips at the other’s fins until they can’t come up for air—a battle that can go on for as long as three days.

Sea Horse
The scaleless, upright swimmer with the equine face thrives better in captivity than in the wild. Attach a low-energy Koralia water pump to mimic the tranquil coral beds of home. Need tank mates? Bring in more sea horses.

Lionfish
With poisonous dorsal spines as sharp as hypodermic needles, a docile creature this isn’t (unless you count how it reacts to things it can’t swallow whole). While your first impulse might be to surround your new lionfish with other fierce species like puffer fish or eels, don’t. Try Niger triggerfish instead.

Discus Fish
A native from the Amazon River basin, it is equal parts exotic and pricey ($150 and up). Naturally inquisitive, some in this group can recognize their owners (they like to beg for worms). Keep the discus swimming with a reverse osmosis filter that softens L.A. tap water.

Ghost Catfish
Consider this an upgrade of those Visible Man anatomy kits. A transparent body makes the fish’s vertebral column, swim bladder, and beating heart fun to observe. Buy this species in numbers—going solo is stressful enough to kill them. 

Puffer Fish
Meet the tiniest trash compactor, which Bernie describes as a “hearty fish with two eyes, a mouth, and a stomach.” By the time a puffer reaches nearly two feet long, its beak can break through crab and mollusk shells. Feed it frozen shrimp, and it’ll be satisfied.

Images courtesy of (in order): (1) Flickr/Daniella Vereeken, (2) Flickr/shellac, (3) Flickr/jayhem, (4) Flickr/sebilden, (5) Flickr/islodelba, (6) Flickr/zenilorac