I am worried about my betta named Rainbow, he has been eating fine but has a lump on his neck area. It has gotten larger since yesterday. What should I do. He is in a divided tank, should I take him out and put him in my other 3 gallon I have that is empty until I know if he will be ok.Here is a picture.

Oh my! That doesn't look good. Is he acting any differently? Is the other betta in the tank acting differently? How are the water parameters, can you test them for us? The one betta might be more senstive to water chemistry than the other.

Personally, I would take him out and keep him in his own tank and do a very large water change in the tank he is in now to protect your other betta. I have no idea what it could be, but its better safe than sorry.

Is it his neck or his belly? It sort of looks like his belly in the photo. Could he be constipated? I know my guys get a belly on them sometimes. When they do I don't feed them for a day and it goes away.

Autumn

I guess you are right it is in his belly. I fed him peas yesterday and only gave him one pellet today. I will transfer him into the 3 gallon right away and hope for the best.

He is either constipated or got a bacterial disease.
What temperature is the water?

Dont feed him for 3 days and see how he is doing

He is transfered into the 3 gallon, still active. Sort of freaked out being in the small tank again, but still looks ok. Will not feed him for a couple of days then give him peas for a couple more days. Hope he is ok. Soon I will be getting my betta from Thailand and want everyone healthy and happy.

Well, I hope for the best. Let's all hope he's just constipated.