I bought some gold fish about four years ago for a science experiment, to see if I could make some lost their color, and it worked. Then i ended up keeping the goldfish which I had for 3 years til i went to college. My sister greatly overfed them for a month straight. Instead of dying, they grew to about half the size of the 10 gallon tank, so I had to buy a 38. I would say they are the easiest to take care of, unfortunately they can live 20 years if you actually take care of them. If you want fish to make babies, get some guppies. It is the easiest thing to do. Get four of the ugly brown ones, the girls, then a cool lookin male and give it about a month. Once the chick gets fat get her in a small tank, like a gallon or something, and she will pop out a bunch. The lifespan is about a year.