Hey everyone, the problem is resolved but it has left me stumped!

Last week I spotted a red spot, similar looking to a cyst, coming out of the side of my gold barb. This at first did not concern me, as the spot that it came up on was a spot that used to be nipped at terribly by the now-gone tiger barbs and other gold barbs I used to have (this gold barb is the only one in the tank and happy about it now!) This spot occasionally gets red (weak I am guessing) but it always goes away.

2 days later, one of the female neon rainbowfish had a very similar red spot/cyst looking thing that was quite raised from the body and looked quite painful. On her little body, I'd say it was about 10-15% of her. Ouch!

So as with any "potential" problem in my tank, I always immediately reach for pure tea tree oil, and dosed about 23 drops in my 50 gallon tank. Did a small water change the next day, and then added 5 more drops.

Within 2 days the spot is totally gone, no other symptoms arose from these fish, nor was there any spots on any other fish.

What was it?