Looks like a tumour. There isn't much you can do about them. I would add some Indian Almond leaf to help avoid infection. I had one that lived well over a year with one before it started to bother him.

 

Hi and welcome to the forum,

In order to correctly identify exactly what the lump is (type of tumor), you would have to have it removed surgically by a fish vet and tested by a lab. That can be very costly.

Perhaps one of the parents carries that in their genetic make up since you have multiple fish that have the growth and they all come from the same spawn.

Dragon scale bettas can be prone to tumors, especially white bettas.

If the tumor has a cauliflower look, that may be lymphocystis, which is an iridovirus. These usually will form on various parts of the betta's body, and the virus will have to run it's course, sometimes taking as long as a year. It's not fatal, just not pleasing to the eye.

Many fish breeders would cull the fish that have the abnormality.

Personally, I would never be able to cull fish, so I will never be a breeder. I just don't have the heart to do so...

I hope this helps!

 

If it is a tumor caused by some genetic makeup, it cannot be contagious, can it?
Because I was concerned, I had been disinfecting the tools I use to clean the male fish tanks. The sorority tank, however, has a few betta fish with tumors and I have not isolated them. I have been changing the waters regularly and maintaining a healthy environment. Should I isolate them nonetheless?

 

I wouldn't.

If it was infectious, they would long have been infected, living together for 7 months.

And the separated males would have been more healthy, following that logic.

 

I wouldn't.

If it was infectious, they would long have been infected, living together for 7 months.

And the separated males would have been more healthy, following that logic.

That's rational.
I'll just keep up with the regular maintenance.
Thanks!