Dystiscus Marginalis (Water Tiger)
The water tiger is far more risky than the dragon-fly larva. It is the larval type of the big cruel water beetle (Dytiscus), and a lot much more likely to be included with live food as it is a totally free swimmer.
There are a number of species, but from the aquarist's viewpoint the following holds true of them all. Water tiger larvae are tough to distinguish when little and mixed with daphnia.
Their technique of attack is various from the dragon-fly larvae, which are content to lie in wait. The water tiger will really chase a victim and sink its two hollow mandibles into it, via which it sucks sufficient blood to kill. It then goes off after an additional victim.
The maximum size is 2 inch, and at that stage they're simple to detect, but you should never permit them to reach such maturity in the aquarium. When they're only 1/4 inch in length they can mingle with young fry, and will eventually kill them all unless they're discovered.
The only real precaution is vigilance. Fortunately, some big fish will eat them, particularly cichlids.