I thought I'd put up a post here of something I learned with my last set of bettas and that I've researched more thoroughly lately. Most people don't seem to much pay attention to the kH of water, basically the carbonate hardness. It seems that if your KH is too low, your pH can crash, thereby causing your fish (and anything else in your tank) some health difficulties. I had some beautiful halfmoons last year, and although i kept them spotless, they both developed fin rot. They weren't in the same tank and I didn't share items without washing them first. I fought with this illness, treating the finrot, for months, and eventually they lost almost all of their beautiful fins and they never grew back. The breeder tipped me off to the KH difficulties and suggested i add 1/4 tsp baking soda to each gallon of water. I started doing that and the "fin rot" disappeared. Upon further research with my new betta and my new saltwater tank, it seems that KH, something we don't often think of, can really affect the bettas and cause illness because of fluctuating pH levels, which we all know aren't good for our little friends.
So if you notice your betta constantly getting sick, you may want to consider the KH content. Baking soda acts as a great buffer and won't adversely affect your little friend if used properly. It's really inexpensive and can very much help extend your betta's life.
Any other thoughts on this topic?