Things You'll Need
Filter with adjustable flow
Water test kit
Aquarium plant (live or silk)
Make sure not to leave the mirror sitting next to your Betta's tank all day long--he may become stressed by having "another male" in such close proximity.
Don't become discouraged if your Betta still does not build a nest. Some males in captivity just never do. Keep your male Betta happy and healthy, and he's likely to make a nest for you at some point.
Bettas are unusual among fish, in that the males are the ones to care for and protect the eggs once they are laid. Male Bettas do this with the help of a carefully constructed nest of bubbles built near the surface of the tank. While there is no way to force your Betta into building a bubble nest, you can encourage him along by making sure that certain needs are met.
Make sure that your Betta's tank is kept at a suitable temperature for the species, between 70 and 80 degrees F. If your Betta is too cold or too hot, he may become lethargic and be less likely to build a bubble nest.
Provide your male Betta with at least one live or silk plant which is large enough to reach the water's surface. In the wild, many Bettas use plants near the surface to help hold their nests in place, and giving your Betta such a plant in his tank may help trigger nest building.
Slow the flow of your tank filter to the lowest setting, or create a filter baffle by placing a piece of aquarium sponge into the filter's outflow to slow the water. Bettas prefer calm, relatively still water, and lots of surface movement will prevent your male's bubble nest from staying in place.
Maintain high water quality by testing the water parameters (especially ammonia, nitrite and nitrate) at least once per week, and changing out 10 to 15 percent of the tank water weekly (You may need to change more if the Betta is in a tank or bowl smaller than 5 gallons--up to half of the water per week).
Place a small mirror next to the male Betta's tank once per day for about 5 minutes at a time. The Betta will see his own reflection and think that another male has moved into the area. This may trigger your male Betta's natural instinct to protect his territory and show off for any females around--which male Bettas do by building bubble nests.