I have had our betta for about 9 months, in a 2.5 gal tank.
I noticed about a month ago, a small nick on his side, that just looked like he might have bumped his leaf hammock and loosened a scale or something. That is what it looked like to me at the time. I have been watching closely, and it looks like this is getting bigger, and now looks like a lump. This picture is not very good, but you can just about see the lighter raised section on his side. (attached). I will try to take a better picture tomorrow.
What is this and what should I do?
I usually do one 50% and one 100% water change weekly using Reverse Osmosis water, Betta fish conditioner and Kents R/O Right to replace minerals. He's been fine otherwise and not shown any sign of not eating or any other strange behaviour.