The great diving beetle larvae isnt called the " water tiger" for nothing. Great Diving Beetle larvae will eat anything they can catch and are said to be even more vicious than the adults. Their favourite prey includes tadpoles and any other insects such as water boatman ( another active predator) within reach. They are also cannibalistic and will eat other Great Diving Beetle larvae. Large larvae in the final stage before pupation, are of sufficient size to even catch and eat small fish.
The larvae are larger than the adult the larvae are around 50mm in length the adults being around 30mm.
As with all things in nature it is a balancing act , the larvea may eat tadpoles etc but being large birds esp thrushes and blackbirds seem to be keen on them ( in my garden anyway) along with frogs, toads and newts , maybe revenge for eating their babies ? . With kingfishers, water and wading birds as well as fish taking them in the wider countryside. The beetles pupate in areas of grass or soil near the pond so i imagine animals use the pupae as a nice little snack as well !
I should imagine that a natural balance will be achieved in the pond as even a large pond can only support so many diving beetles and dragon flies !