What were your water parameters, and what is your feeding schedule, amount, and the water change schedule and amount. The upper photo does indeed look like hydra, and they usually flourish in a tank with high levels of dissolved organic matter. The lower pic, I have no idea, but I would guess at snail eggs.
Hydra are related to anemones, and have the same stinging nematocyts that the anemonies do. They could cause irritation to the fish in high enough quantities, which could conceivably break down their immune system enough to allow pathogens to gain a foothold. Do you have any pictures of the fish? Also, what size tank, what kind of fish, and how big are they?
I know it's alot of questions, but we need this info to straighten this out!
