Partner set up a very small aquarium last year with an almost fully self sustaining "pond" ecosystem. Plants, snails, a few guppies, some tiny shrimp, algae. No pump, no continuous lights (just a couple of hours of very low wattage LED a day).
They did this for teaching and fun for and together with the eldest of the young family we took care off for a few years.
And one day, a mosquito made it past the fly screens and was buzzing around the living room.
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