Nearly all water beetles are carnivorous, and it is very unlikely that they’ll find their way into the aquarium if reasonable care is taken to

There are numerous different kinds of thread worms. They’re recognised as hair-like creatures, wriggling their way about the tank with eel-like motions of the body.

The great pond snail, being a general feeder, has a strong leaning towards animal matter, and if put into a tank containing hydra will soon

Again we have a pest that’s not truly an enemy. On rare occasions leeches might attach themselves to fish, but this is the exception instead