So a somewhat menacing looking blackness has developed on some of my tilapia. Here are two images of one of them:
The back story is as follows:
I was running an aquaponic system in my yard this summer with about 120 red bellied tilapia split between two 100 gallon bins with a 100 gallon sump and ~ 1000 gallons DWC plant bed. Unfortunately about a month ago my roommate decided to clean the system, but didn't follow through, leaving things stirred up with very little water flow. By morning, there were a number of dead fish, and I continued to pull out dead/dying ones over the next 10 days. They appeared to have contracted saprolegnia, as they (the dead/dying ones) had white cotton like fluffs protruding from their skins.
After it was all said and done, I had about 40 fish left, all of whom seemed healthy and showing no symptoms of the fluff, so I decided to move them to an indoor aquarium, a 55-gallon tank with a fluval u4 submersible filter, a heater (keeps us around 80), and plenty of air/turbulence.
I am well aware that 40 tilapia in 55 gallon tank is very much a short term solution, but I intend to try my hand at breeding out 3 or 4 pairs of them and I have yet to try and identify the genders. But that's an aside. The tank is rather clean and everyone appears healthy and eating.
Everyone except a few, that is. I noticed when bringing them indoors a couple weeks back that some of them have a black underlining on their bellys. It is very pronounced in only a few of the fish (say about 5), and does not appear to be spreading between fish. I was not able to find anything like this on the internet, and a very knowledgeable aquaculturist friend of mine didn't know what it was either. Perhaps someone here can help? What is this stuff, and should I be worried?