Possibly ID this growth

[Oldreeferman]

Kind of strange bump appeared under the RH side fin today, its about 1/8" dia. maybe a bit larger. Its on my adult cardinalfish that ive had about a yr. now.
Fish is acting a bit skiddish more then usual.
2nd pic shows the outward growth, just fed & its acting normal so far & now hanging out again.

 

[mcgyvr]

Its a zit.. Pop it

 

[Oldreeferman]

11 days later & it completely healed up on its own if anyone is interested, would have been nice to know exactly what it was since ive never seen a growth like this one on a fish before. Scoured the web but to no avail on this odd growth. Ive not changed or done anything differently in the last 6 months so im stumped but happy it did not effect any other fish. Maybe a fish can get a pimple!