How can you tell a betta fish has a tumor?
If the lump is covered with scales, and the scales are a
continuation of their normal scales / skin(?) then it is probably a
tumor. It is easiest to check this by using a torch to directly
shine light on the lump, but please don't shine it in their eyes!
If it is not covered with scales, then it is likely to be an
It is quite possible for Bettas to live a long-ish and happy
life with a tumor, but sometimes the tumor affects more of their
body than you can see, and they pass away.
Tumors are very common with Bettas, due to inbreeding and
hormone therapy when they are young to make them grow quicker, to
make more money.