Hi Dorothee, and congratulations on the fry.
Sorry im a little late to this forum, but ive been on holiday.
But to take the topic back slightly, I have also had the strange structure in my paro tanks. It has only occurred in my breeding tanks, and only around the nest site. I have never had it in my female only tanks, or tanks with none breeding fish.
Is this the same as other members observations, or is it just a coincidence?