Animal to go in a 30 gallon tank?

Ok, so I have this 30 gallon tank just sitting in this room with my two guinea pigs and two rats. I would like to put something in it that's easy to care for and that has a lifespan of 10 years or less. I don't want a spider or something that has to be fed small rodents. Just something that would make a really good pet and that wouldn't be terribly hard to care for. Any suggestions are appreciated! Thanks!