Hello!
I don’t understand that panic about those unplanned guests (o.k. I wasn’t pleased about my little dragonflies in my new tank, but I’ve got them out just by searching them out in a nearly emptied tank)’.
Once I’ve also had hydra in another tank, we tried to get them out with wooden sticks. But they vanished before I could bring some to a teacher of biology who wished to get them.
Also we got planaria as unwished guests, I catch them out with a meat trap.
I would never risk some chemical substances in the tank.
I would rather make a new tank for the fish and shrimp than to use any poison!
I guess we have much more unknown guests in our tanks than we want to know.
And nevertheless, it’s always a way of biological balance …