75F is good. in a tank that big you won't want to do weekly water changes so you'll want it filtered and heated to keep it a constant temperature.
IF YOU JUST GOT THEM YESTERDAY DO NOT SPAWN THEM. since you posted this, i'm assuming you're new to bettas. do not spawn until you have read up on it. if you just release them when the male has a nest and the female isnt ready, you'll get a dead fish. you have to condition them, you can't just throw them together. they arent livebearers, they arent easy to breed. another thing is, everyone will tell you READ ALOT before you even attempt to breed. if you attempt it without reading up on it you will end up with a dead fish and an unsucessful spawn.