ahh! im trying to think of sumthin but a water change i dont think will get rid of em. ill be back, lemme research sumthin......TO THE GOOGLE!
BACK! ok, they recommended salt and a current, hydras are strictly fresh water, even thogh they have relatives that are marine, they cant tolerate salt. hydras also have trouble liveing in moveing water, they can not actively move on their own, they are very much like jelly fish and have no way to propell themselves throgh the water, adding a light current will make it difficult for them to reproduce and catch food.
i know baby bettas dont take well to changing water conditions and the salt idea is out, i dont recommend changeing out water to adding any strong meds. all i can say is add a bubble stone to get a current going. once the fry are free swimming u will have to turn it off thogh as they will be blown around quite a bit.
and a quick question, is the father in the tank with them? if he is careing for them they will be much better off as he moves them around. im sry i couldn't be more help i dont know much about micro bacteria in aquariums thogh im sure i have plenty in mine.