Hi Peter,
thank you for your prompt and detailed response.
Meanwhile, the larvae swim free.
However, I have chosen a different way. I have not removed the parents, but very cautiously given the cave with the larvae in a small plastic basin. With success! B)
To your question, yes it is filter material which is inserted into the hole. I once had made bad experiences with Betta and flowerpots. That’s why I’ve used it out of habit. But you’re certainly right in Paros I do not need it.