I have a betta and it has been about 3 months from the purchase date. He recently can not keep his food down. I feed him small pellets normally and he will eat one and spit it back out. I then thought that maybe right now he just can't swallow somthing that hard. A little later I fed him a blood worm (thinking that since it is soft it will go down easier) he eats it and spits it right out again. I then tried feeding him fish flakes and the same thing happened. He also is not as active as he was before. He now just lays on the bottom and periodically come up for air. I changed the water because of all the uneaten food. What is wrong with my fish?