Ok, this is what you do:
1. Go to a healthy pond or swamp, ideally with lots of vegetation and an open area next to it.
2. Set up a big white sheet and a bright (preferably ultraviolet) light hung from a rope between two trees. Lay another sheet on the ground.
3. Wait until nightfall. Watch as dozens of aquatic insects fly to the sheet. Don't get bitten by a giant water bug.
4. Collect as many Cybister, Dytiscus, Acilius, Thermonectus and other large predaceous diving beetles as you can. Bring them home.
5. Plop them in the tank, and watch them devour the guppies. Once they have run out of guppies, you can feed them basically any kind of flesh including pieces of raw fish, shrimp and chicken as well as live or dead insects such as roaches. If they have enough food cannibalism should be minimal.
6. You now have a tankful of large, active, voracious and above all unusual aquatic insect pets! Since they're pond- dwelling air breathers, they couldn't care less about water quality so tank cleaning is mostly a cosmetic concern.