Tomoko,
If I had your problem, I would make sure I had enough Java moss in the tank for the fry and stop feeding them BBS for a few days and add the Fluke Tabs then. I'll bet it will get rid of the most of your problem. Also I found that if I them feed the fry sparingly a few times a day instead of a big shot all at once, the Hydra doesn't come back. I haven't had it for a few years now.
Don