They are very beautiful!!
As a word of warning: I think I've seen those tanks before and there's something wrong with the gravel that pollutes the water. Is that gravel the stuff that came with it? That could be responsible for the death of one fish.
Oh, and often you can't see ammonia at all but it can be deadly to fish which is the reason that water that "looks" fine is not always fine at all. I'd try to upgrade the tanks to at least 2.5 gallons each, and until then do a water change at least every other day on the ones you have now. What temperature is the water? Bettas are tropical fish...