The dragon fly larva
It is similar to the Water Tiger except that it does not swim much but lies in wait for its prey. The victims are merely held by the pincers and are then destroyed by its hard mouth, which can do amazing damage even through the study scale protection of quite big fish.
It is different from the two described earlier, being very much smaller and capable of harming only fry. Its presence is betrayed by a group of bunch of thread-like streamers or hairs hanging in the water; these are the tentacles. Each hydra is roughly 3/4 inch long, having about half a dozen tentacles spread out like a star fish. Since it invariably seems to multiply rapidly, ways have had to be found to remove the vast numbers other than by emptying and cleaning the pond or aquarium.