Think about a Siamese fighting fish. They are solitary, so just one per pot is perfect. They don't need filtration (as long as the pot is reasonably clean), aeration, or much space at all. They are tropical, though, and do need warmth. They won't root around or hurt your plants in any way.
Their natural diet is mosquito larva, and they WILL keep your pest population down. I will often bring one of my indoor fish out to a pot or tub that has larva in it, and leave it out there for a day. At day's end, ALL the larva will be gone.