Just stick it out for a few days. I dosed no planar a few weeks back to kill planaria and at the end of day 3, I still saw some wigglers that struggled on the glass. Kind of freaked out but just followed the instructions and so far I am planaria free. If the hydra survive, repeat the process in a week or two with a big higher dose. As a reference, I went full dose in my tank with Crystal Red shrimp and only 1 of them died out of 20.