I HAVE A SICK FISH. . .HELP!

Question
I HAVE HAD MY BETTA FISH FOR ABOUT A YEAR NOW. HE WAS DOING GREAT AND THEN I NOTICED I NEEDED TO BUY HIM MORE FOOD. MY MOM WENT TO PETCO AND TOOK THE BOTTLE WHERE THE FISH CAME FROM SO SHE CAN GET HELP GETTING THE RIGHT FOOD. SHE ASKED FOR HELP AND SOMEONE GAVE HER THE FOOD FOR THE FISH WHICH WAS DIFFERENT FROM WHAT WE USED TO GIVE IT BUT SHE SAID THAT IT WAS THE SAME THING JUST IN A DIFFERENT CONTAINER. EVER SINCE THEN, HE HASNT BEEN SWIMMING THAT MUCH. HE JUST STAYS AT THE BOTTOM OF THE TANK AND JUST COMES UP FOR FOOD AND AIR. HE HAS BEEN LIKE THIS FOR TWO WEEKS NOW AND I DONT KNOW WHAT TO GIVE HIM. TODAY I NOTICED THAT HE IS GETTING A LITTLE BIT OF YELLOW UNDER HIS BODY BY HIS GILLS. WHAT SHOULD I DO?  

Answer
Hello Elizabeh-

A change in food should not cause any sickness in your fish. As long as it is betta food (I'm assuming you use pellets instead of bloodworms) he should be fine.

He may be picky at first, but he symptoms he's showing make me think that his water may be a little too cold. Check his tank and be sure everything else is the same. If so, just give your fish some time and see if he improves.

Hope this is useful,
Amber Worman