Notonectidae is a cosmopolitan family of aquatic insects in the order Hemiptera, commonly called backswimmers because they swim upside down. They are all predators, up to nearly 2 cm in size.
Backswimmers are predators and attack prey as large as tadpoles and small fish, and can inflict a painful "bite" on a human being.
They inhabit still freshwater, e.g. lt lakes, pools, marshes, and are sometimes found in garden ponds. Although primarily aquatic, they can fly well and so can disperse easily to new habitats.
How to raise -
The big problem with Notonecta is cannibalism. As soon as one of them got a little bigger it starts to eat the other ones. End of August I have two fully developed ones left out of 12.
My suggestion is to keep them in separate containers, no more that 3/4 in 5 liters grouped and regrouped periodically by size.