What fish would you recommend for a beginner who wants an aquarium?

Actually, I raised guppies myself and the fish happily occupied exactly one aquarium. The population takes care of itself with little or no help from the fish-keeper. When there is more room, they breed more and eat fewer fry. When there is less room, they breed less and eat more fry. Plus, the guppies I raised were originally purchased at a big box store, turned out to be Mosaic Deltas, and thrived for several years without artificially heated water. The only heat they received was from the ambient room temperature, which of course, was warmer in the summer and cooler in the winter. When I finally felt a little guilty and went out and bought an aquarium heater, I put it in the water and within a few hours the fish seemed sluggish. A few hours more, and they were definitely looking stressed. The heater I used had only one temperature and stopped running when the water hit that warmth level. When I took the heater out of the tank, the fish quickly recovered and resumed their happy little lives. This is why I insisted that guppies make good starter fish. I emphasized in my original post that you should buy feeder guppies, rather than the more exotic breeds, for the purpose of 'starter' fish in a first time aquarium. Some types require more care than others and should not be purchased unless the fish-keeper is a bit more experienced.