strange growth on female betta - help!
So I've noticed over the past week or so that aquaneesha, one of my three female bettas, has developed a strange lump of some sort on what I would describe as her "chin."
I don't know if I could successfully photograph it, so I will just try to describe it. It's fairly large for the size of her body, but it doesn't seem to cover her gills or inhibit her breathing in any way.
About ten days ago, I changed out a little water and must have filled the level a bit too high for the divider, because my other female betta jumped into Aquaneesha's territory and they squabbled overnight. In the morning I found Piper, the one who jumped the divider, with pretty severely nipped and shredded fins. Aquaneesha apparently defended herself well and had no real nips or rips to her fins.
I'm wondering if this is an infection on an injury she may have received from that encounter. The bump began as a smaller lump that was the same color as the rest of her body, but now it is bigger and white-capped, almost like a giant pimple.
Her behavior doesn't seem particularly affected.
What medication would you recommend I treat it with if it is an infection or injury of some sort?
If it is a tumor, I guess there's not really anything I can do, right?
Thanks for your help. She is in a 5.5 gallon tank (with a filter and heater) with two other female bettas. I intended to have a peaceful sorrority but that didn't happen, so they now each are divided into a third of the tank. The other fish are very healthy and colorful, so if it's an illness it isn't spreading.
I will admit that I don't have a test-kit for the water levels, so I don't know what they are, but I do my best to keep up with regular gravel vacuums every couple weeks, and I change out smaller amounts of the water more frequently. I could get the levels tested when I go to get medication if necessary - would there be anything in particular that would be abnormal and associated with her condition?
Thanks for your help!