Tumor?

I would love to know why all of my fish do great for a period of time and then they all get problems at the same time. Is this a tumor on my betta? And if so, what do I do? He’s behaving normally.

That does not really look like a tumor to me, it looks more like some disease or external infection in my oppinion, could you possibly provide a better photo?

That area looks to discoloured to me to be a tumor but I am not exactly sure.