Are both of your bettas females? Males and females should not be put in the same tank together except at breeding times and then only long enough to breed and then seperated immediately. Two males shouldn't ever be together. I seriously doubt that you would still have them if they were males.
For a tank that size with 2 fish in it, it depends on the stage of the Nitrogen Cycle you are in. Do you have a Master test kit and can you send us the levels of ammonia, nitrite, and nitrate in the water? We really need to know how long the tank has been set up and if the filter is cycled or not.. If you are having any ammonia or nitrite or nitrate problems in the water the tank will have to be changed much more frequently. If you have a new or uncycled tank you really need to read the following article about the Nitrogen Cycle.
I would still recommend a 50% water change at least 2 times a week in a tank that small. Small tanks are much harder to keep and much more fragile to maintain than larger ones. Ammonia builds up much faster and I have known toxic levels to develop almost overnight in an uncycled small tank. Even in a cycled small tank, the balance is very hard to maintain when there are really more fish in there than recommended. I am sorry to say that the recommended size for ONE betta is not less than 5 gallons. Since you have 2, your recommended tank size would be in the 10 gallon vacinity. Bettas tend to become aggressive toward each other when they are crowded so please watch for any injuries on the bodies or fins of your fish.
I do wish the best for you and your fish. Please do keep us informed on how you are doing.
my tank is made to be able to have two male Betta's it has a piece of some sort of black film that you can remove and put back on so, they can not see each other. and Ben and jerry are doing just fine and I changed the water today
Is this a filtered and heated tank or a Betta container?
Bettas are Tropical Fish and a lot of the containers that are being sold today as "made for Bettas" are not going to be good homes for them as they are not filtered to remove the ammonia, nitrites, or nitrates. If this is the type of tank you are talking about then they are going to have to have changes of about 50% every other day to keep from dying of ammonia poisoning. They are also going to have to have a 25 watt heater in the tank to keep the water at a steady 78 - 80 degrees F as they are as I mentioned Tropical fish and prone to ich, velvet, fin rot and a few other diseases that tend to come around more frequently in what we call "cool water" (less than 78 degrees). Bettas are also prone to constipation which happens more in fish when their metabolism is slower, like when they are kept too cool. When their body temperature is kept up at 80 or there abouts, the chance of them developing constipation is less, but you also have to watch how much they eat and do not overfeed. (2-3 pellets per fish twice a day OR 6-7 freeze-dried bloodworms twice daily) If you give them one piece of food and watch them chew it up before you give another piece it makes them eat slow enough not to get all plugged up. It takes time but believe me, the dealing with a constipated betta (bettas can die of constipation) is not a fun experience as many of our betta owners can tell you. Besides, bettas develop their personalities by spending time with you, and this is an excellent time to talk to your little betta and get the personality going and soon he will respond and feeding time will be a special time you will both enjoy every day.
Sorry I just reread the post below that said your tank was filtered and heated. Please do look into the feeding thing though. It has been a long day and I did not go back to the other posts, I do apologize.