I can assure you the Betta does not think of you as a mate. They are not stupid and do know a female of their own species. Your Betta has simply reached sexual maturity and is building his display home in the hope that if a lady Betta comes along he can convince her to make fertile eggs with him under it.
it looks like they have a big bubble in their side
It probly is a female
Female Betta fish do not make bubble nests.
check the PH in the water,
he is unaware that you just let a another fish in and wants to attack her for his territory,but if he starts making a bubble nest that means he is ready to spawn(mate) so put her in
BETTA Fish i think
http://www.firsttankguide.net/betta.php
Betta fish derive their color from refraction within a layer of translucent guanine crystals.
The betta fish will see the mirror and then will be curious about it, because the mirror reflects the betta fish and they might think that it is another fish, so the fish will act differently.
The male will make a bubble nest when the female is about to lay her eggs.
they can. but it is VERY rare. usually you would see a bubble nest.
Place a female in close proximity where he can see her.