What causes ulcers in fish?

[Navfish]

I searched it up online but couldn't get a clear answer. I have some guppies going through this, maybe due to switching tanks but im not sure?

 

[GaryE]

Ulcers often go with Mycobacter marinum infections. If the ulcers are sort of rectangular in shape, odds are it's that. It takes skills with a microscope and dead fish, or weeks of culturing in a lab to be certain, but it is a disease common in farmed guppies.