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Guppies

Honestly, goldfish and Siamese fighting fish may be beautiful and low-maintenance pets, but they're … boring. And actually, while the initial investment may be more, an actual aquarium with a filter needs cleaning much less often than a goldfish bowl. Populate a 10-gallon aquarium (most come with integral lights in the hood) with a few guppies, and let the fun begin. They'll mate, have babies, eat the babies, watch the babies grow up and mate, etc. Adding a few other fish or aquatic frogs won't set you back much more and makes for an even more interesting social scene. (Add one kind of fish at a time to see how they get along.)

Photo by Falashad