It all depends on the temperature of the water in the container, due
     to the temperature of the room the betta is in.  The betta should be in
     at least 72 degrees water. IF the room he is in gets lower than that, a
     form of heater needs to be present.  If you only have a few jars, you
     can put them inside a tank, fill the tank with water to at least
     halfway up the jars, but not more than 3/4ths the way, you can heat
     the tank water and thereby heat the jars, make sure the temperature
     does not go higher than 85 degrees.  This is an easy way to heat a
     few jars. If you have more than a few, you might think about heating
     the room they are in.Bettas do not do well below 70 Degrees, To cold
     and they will get sluggish and loss color, and even die.