Gouramis can be a pain to raise ( easy to breed tho.
) and with that, you need to make sure there is enough food for all those fry and constant clean water once the labrynth has developed. They do also produce a growth inhibiting hormone (somatostatin) which can greatly effect all but the few largest and strongest fry. ( You can read about this here:
Somatostatin and somatostatin receptors in fish growth. - PubMed - NCBI) Constant clean water dilutes the hormone thereby making more fry available to grow. The catch to this is that you then need larger tanks or more tanks to grow all these fry in. So the fact that you had a few fry survive is "normal" for a typical spawn in a smaller aquarium ( and yes, a 30Gal would be considered a smaller aquarium.
) The other option is to separate the largest fry when you see a noticeable size difference but that just makes it possible for the next few to do the same thing to their siblings.
If the reason is in fact Hydra, keeping things like snails in the tank is probably the source for them. ( Live plants as well.) Mollusks of all types are known carriers of many diseases and really need a long quarantine time to ensure that whatever is in them is out of them so they don't release anything into the tank. This is why I choose not to use snails anymore. Bare bottom tanks and a good siphon hose works 100% of the time.
Pearl Gouramis are usually very good parents so the male eating the fry seems unlikely, but you never now.
This is again why I remove the parents once the fry start to swim. The rest of the job is not really necessary for the parents to be there.
Hope this helps
