Females do tend to be smaller than males. How big are the bowls you're keeping them in? Do they have heaters? It is a common misconception that bettas can be kept in very small containers. They are tropical fish and need consistent temperatures around 78-83 degrees to be comfortable, healthy, and active. Also, just like other animals, when confined they develop neurotic destructive behaviors like glass surfing and tail biting. Please keep in mind that fish constantly excrete toxic ammonia from their gills--this is kind of like the fish version of urine. In a closed system, this ammonia quickly builds up and can actually burn the fish. If the bowl is a half gallon, you should be changing 100% of the water every day. If it is a gallon, every other day. However, even if you keep the water pristine, for the other reasons stated above, this is not a humane way to keep a betta permanently. I strongly advise keeping your fish in homes that are at least 2 gallons because most heaters are designed for 2 gallon tanks, but the bigger, the better. Larger tanks are lower maintenance and provide more environmental enrichment.
Also, aside from mere logistical reasons for not breeding your bettas, there are also ethical reasons. There are fish dying on shelves every day because not enough people want to buy them. Why would they go out of their way to get the same fish from you if they can just go to the store? You might end up with dozens, to hundreds of babies. Are you willing to feed them for months until they reach a size that people will want them? Are you willing to house half of them separately and maintain all of their tanks, assuming half are male? Will you have the stomach and the experience to kill deformed or undesirable babies humanely? Think about it, you're the one who's going to be bringing new life into the world. It will be your responsibility to take care of them and ensure they have the best future you can manage for them.
Don't breed animals just because you can, you do it for the purpose of selecting specific traits that will contribute to the overall well being and value of the species--if fish mills are cranking out the same kind of fish you have, then you're not doing anything for anyone. If you want to be serious about breeding bettas, you need to research your method, decide on a goal, then buy a quality pair from a private breeder to start you off.
Last edited by Adastra; 07-15-2010 at 07:25 PM.