Tank setup for spawning in relatively easy, and the rules are simple, NEVER use anything less than 10 gallons. I use ten gallon tanks myself, I fill them about 1/3 of the way and put duckweed, java moss, apogneton and sword plants in the tank. I let the apogneton and sword plants float and alone with the duckweed they create a biological canopy that assists in nest building and makes the environment more “Friendly” to the bettas.
Many people also put styrofoam cups cut in half and taped to the side of the tank in and bettas do tend to choose that to build their nest. The water temperature needs to be as close to 80 as possible and it needs to be absolutely stable. I also recommend adding AquariSol to the tank to prevent infections and parasites, especially if the spawn is sucessful. You must use a fully submergeable heater, and you need to affix a thermometer to the tank (below the water line),
It is ABOSLUTELY IMPERTIVE that your tank temperature and chemistry remain constant – our air conditioner broke this summer causing the air temperatures in the house to be able to rise into the 90s – that caused the tanks to heat up – over the period of 48 hours there were up to 12 degree tempeature fluxuations in our spawning tanks (6 up, then back down) and we lost fry because of it, it was a very large disappointment and very frustrating because we could not afford to get a new air conditioner. Now that you know about conditioning you can move on to Spawning.