A bubblenest is when the male makes saliva bubbles on the surface of the water. It is something they do when they are ready to mate or when they are EXTREMELY happy. I would say that Julian is extremely happy as he has not been prepared to mate and there is not a female around. If they love where they are and what they have around them they will blow bubbles all over the top surface of the tank. The first time I saw one I did not know a thing about Bettas and I kept wiping it away, but Azul kept replacing it and I eventually let him keep it. He was just ecstatic over his new home.
It is a very nice thing to have happen.

Congratulations, gammerus!
Rose
