i had some in my aqueon evolve 2, a shrimp tank, dosed an eyeballed >.1g of safeguard dog deworme (i think fenbendozale or whatever). Didnt kill any of my shrimp and have not seen a hydra.
I did this too for my 5 g -- I took out my ivory mystery snail, visually approximated 1/20 of the little packet and plopped it in. No more hydra or planaria.
Next time (I think I saw a Planaria yesterday) I will try to crush it with a spoon, stir it into a cup, and get it to go into solution first.
Take out Purigen and carbon before adding!