Bette/ strange behavior

Question
Hi, Kat, my betta are acting quite strange, for a few days not they aren't eating and stay at a vertical up and down postion???  They will stay like that for hours at a time!  One of them was more aggrestive than the other, we call it Chloe, and the other, Sebastian.  My boys are very beautiful, and I would hate for something drastic to happen to them.  They are in a divided tank and I clean it once a week.  Could you ease my mind????  Please??

Answer
Hi,
My first thought would be lack of oxygen. Their tank must be filtered, but not have a strong current. Make the filter break the surface of the tank a little more, as that will give them a bit more oxygen, but do make sure they aren't being blown about by the current.
Feed them a variety of food, including bloodoworms, brineshrimp and daphnia, as well as their normal flake/pellet food.

Also, if they are both males and can see each other 24/7, they may flare too much at one another and damage their fins. I would suggest putting some sort of opaque plastic or something  between them for most of the day, but allow them to see each other for about an hour per day.

Bettas need at least 23 litres each, so your tank would have to be about 45 for two.
Remember, never do 100 water changes as you will lose all the beneficial bacteria in the tank. Just do 30 percent once a week.

I hope they get better,
Kathryn