For both tropical and coldwater fish, the great danger when catching live foods for a pond or aquarium or when procuring plants from untried sources is that the water enemies will be introduced as well. These are small aquatic insects which can kill or harm baby fish and in at least two instances even medium sized adult fish. The main water enemies are the Water Tiger, Dragon Fly larvae and Hydra.
The Water Tiger
It is a long and thin insect, shaped rather like a caterpillar, and has two vicious pincers with which it can hold its victim and through which it can suck its blood. It grows to 1 1/2 inches or more but probably its most dangerous size is half grown as it is then difficult to detect. A good swimmer, it can tackle and kill fish twice its own size.