Be careful, a heater in anything smaller than a 5 gallon can be very dangerous! You can burn your beta. Also popeye is often a result of inadequate water conditions. Meaning your water is dirty, I mean in most cases that's the case. A container with less than 1 gallon needs to have a 50% water change 2-3 times per week. To avoid future problems you NEED to get a 2.5-3 gallon tank. Bigger is even better. If I were you I'd get a 10 Gallon tank and get a diffider in the middle (they sell them in pet stores) this way you can keep both betas in the same tank while having them seperated.