Q: I put two male bettas in a 2.5 gal tank w. divider, one of them is very happy (bubble nest, swimming, eating) the other will flare but then lays on bottom only swimming to get air.  Is he sick?

Question submitted by Ginnie, Fort Myers - Florida

A: HI there Ginnie. I noticed you have a last name that sounds Lebanese. I spent several years  in Beirut and still speak the language fluently. (Ahla ou Sahla fiki :)  ). I am glad to be able to help you today. Your question is a good one because it is going to give me a chance to go off on a very informative and useful tangent. :). (and I loooove tangents ;) ).
bulletYour betta flares. This is usually a good sign. Really sick bettas will not flare. 
bulletBut you also add he is not swimming and mostly lays at the bottom. This indicate a betta that is either old, sick or has swimbladder problems. I am going to rule out sick, so that leaves us with either old (how long have you had it? If more than 1 year, your betta is probably 2 years or older - if you got it from petstores that is).
bulletOr it could have swimbladder problem. Is its belly swollen? Do you feed large quantities of food at each feeding? Does he appear to swim with difficulty as though it is an effort to go up to surface?

Now I will go off my tangent :): Divided tanks are cool because they can hold more than one betta. However they present some risks: Your two bettas are sharing the same water and living condition. If one gets sick, he will infect the other. In a way, two bettas in a tank are like two convicts chained together. You'd better make darn sure both your fish are very healthy at all time. If one of them shows ANY sign of sickness, it should immediately be removed and BOTH bettas should be treated. At least I would add a good disease preventative to the healthy looking betta, such as Bettamax, just to be on the safe side.  This problems is especially troublesome when you have a large betta barrack, or drip system, or anything that houses a large number of bettas all sharing the same water. These are bound to "crash" on and off, where a disease will take hold of a few bettas and them quickly spread though the entire colony and decimate your stock. Owners of such systems must be very attentive and stay on top of their apartment complex tenants LOL, big time. True, UV sterilizers help some but they do not eradicate all bacteria. Regular water changes, proper maintenance, quarantine of incoming fish etc are a must for any such large systems. 

Now back to you. I would remove the 'not so healthy' betta and keep him in a large jar, add some bettamax to his water for a while. Find a more suitable and active companion for your other betta. Remember to quarantine any new betta first before adding him to your dividing tank, so it doesn't bring mayhem with him! If you want your bettas to remain active longer, you might consider purchasing them from a breeder instead of a pet store. You can get them young that way (3 months old instead of 1+ year old). They will last you longer. Goodluck!