When I was a child my father and I kept a "natural" aquarium. We had an assortment of fish through the years. We would either seine or kept really small fish we had caught. When the fish became too large for our ten gallon aquarium (we had four tanks..2 20 gallons and 2 ten gallons, but only one for the "natural" critters.)we would free them in a local body of water and go looking for a replacement. Keep bluegill and largemouths mostly, but had a few pumkinseeds and a couple of very small blue cats at one time. Also an assortment of other critters...crawfish, tadpoles, an occasional salamander. There was a stream near the house that was loaded with mosquito fish that were the main fodder for the bass and bluegills. One can learn alot from observing nature..even when confined to a ten gallon tank..