Male Betta Built The Bubble Nest, Now Won't Eat.

All of a sudden, he started to build the bubble nest after I moved him from the 55 gallon community tank to a 3.5 gallon living alone.
He seems to be happy, ate a lot, but a couple of days ago, he started to build the bubble nest, and stopped eating...
I bought a female for him that day, but only a juvenile was left on the shelf, and she was half inch shorter than him.
I read some and found it was not recommended to put two betta that were not close in size together for breeding, so I moved her back to the community from the isolated container put in male's tank earlier.
The problem is he is still not eating and food grow mold.
What can I do now? Anyone had the experience to share? Much appreciated!

 

Possibly try a different food or maybe a couple flakes to see if he goes for it. He could still be settling into his new place. A bubble nest is a good sign he is happy. I'de test the waters to make sure they are safe as well.

 

Make sure to remove the uneaten food. Are you trying to breed them? Are you prepared for raising the fry and separating them as they age?
I agree about trying a different food. Some bettas are picky. Mine aren't, though... They're little piggies. All my fish love Bug Bites. Since it's only been a few days, he's not going to starve soon. Is he still swimming and behaving normally aside from not eating?

 

Possibly try a different food or maybe a couple flakes to see if he goes for it. He could still be settling into his new place. A bubble nest is a good sign he is happy. I'de test the waters to make sure they are safe as well.

I’m now a bit confused.
He was in the 55 gallon with molly and platy fish. Probably ate a lot fry that I was unaware of, this was the only thing I was suspecting for this not eating issue.
He was doing fine in the past few days, except for not eating anything that I know. I tried to put a female in his tank to cheer him up, he was behaving normal, territorial and aggressive. If I can I would breed them but it was not the first priority. I want to keep all healthy and happy.

 

Make sure to remove the uneaten food. Are you trying to breed them? Are you prepared for raising the fry and separating them as they age?
I agree about trying a different food. Some bettas are picky. Mine aren't, though... They're little piggies. All my fish love Bug Bites. Since it's only been a few days, he's not going to starve soon. Is he still swimming and behaving normally aside from not eating?

He was a little picky. When was in the main tank, he shared bug bites with molly and platy. That was not Betta good and they prefer different ph environment so I eventually took him out. Now I literally have no idea. I don’t want to feed him fry so I will starve him for a few more days and observe for his behavior.