You mentioned magnification, just how magnified are the images?
Nerites can still lay eggs in Freshwater, but they won't hatch (or survive) without being in Brackish water.
Look here at these magnified images of Olive Nerite eggs
Applesnail.net ? View topic - Olive nerite breeding/veliger pics in freshwater
They sort of look similar, but the shape of the egg capsules are a bit different, your pics look more rice-grain shaped. Still possible though.
Pond/bladder snail gelatin egg sacs look quite a bit different.
Fish eggs are 1 fish per egg (so not multiple eggs/babies within one sac) as far as I know.
Again I don't know just how magnified the images are, but look at "Nematode eggs" or "Parasite eggs". They can look very similar to your images, but those are quite magnified. If parasite eggs are what you have, I would look at getting Anthelmintic (antiparasitic) meds, such as Levamisole or Fenbendazole. Praziquantel or Metronidazole might not be effective depending on the parasite.