Betta with a heater and sponge filter in a 5 gallon tank. Much easier since they don't produce as much waste.

The thing about fish is their environment is almost completely enclosed. The owner must provide for everything right down to the amount of oxygen in the tank. Basically having fish is playing god to an animal because its by your hand that they will so much as breathe. At least with a dog or cat they can breathe the air on their own.