Phantom Midge Larva
This transparent larva is present in most of the pond I visited, even ones populated with fishes. It stays still in midwater but when disturbed is able to swim very fast fora short distance.
It is a predator therefore it is better not to keep it with other small creatures such as daphnia or ostracods, unless you really want to feed it to get a useless mosquito.
This transparent larva has a very scary and hellish aspect.
This nasty little critter is going to grow in a even more nastier blood sucking monster