Spot on Betta tail

[Kearmstr]

Francis has an odd growth near the base of his tail. This came on within the last week. He’s acting and eating as normal. 

I’ve treated him for fin rot twice in the last 6 months with Kanaplex. He responded to it well. His only other tank mate is a pleco and they ignore each other. 

Any thoughts are appreciated! May try a salt bath later (in a separate container) and do a 50% water change.

Ph- 7.3
Ammonia- 0
Nitrate- ~10
Nitrite - 0
Gh - 150
Kh- 40
Temp - 78

[Colu]

Would treat with aquarium salt 1 table spoon for 2 gallons for 1 week and monitor