The picture is not perfect since it is a bit dark under the bubble nest.
I finally have them mating. Even though the male and female Opaline Gouramis are in the same tank they just fight and never go into mating mood. Then I put the female in a net trap and put a piece of styrofoam on the surface to help the male to build the bubble nest. Almost immediately the male runs into mating mood and starts to build the bubble nest. At the same time the female starts hiding. I used this trick twice and it worked twice!
It is the second day since the male builds the bubble nest. I saw them mating in the morning and early afternoon. I didn't see any mating activity in late afternoon so I get the female out. I am sure of the mating, but I don't know whether there are eggs in the nest. There seem to be some yellowish patching in the bubble nest (looking above from under the nest), or it could be just my illusion. How to verify whether there are eggs before hatching?
BTW, can I take the bubble nest (and the nest) away from the male for hatching? I am afraid when the fry start free swimming they will definitely be eaten by other fish in the community tank.