Hey everyone, I've had my 55 gallon set up for probably about a month, it is set up with a DIY sump and overflow and such, I have had low to nil ammonia since I started the tank, and it is still at well under .25 from what I can see with my liquid test kit, I have seen nitrates in the tank, so it seems that it is cycled well.
It houses four goldfish, three are under 5 inches, two comets and one common, and one is probably 7 inches and a comet.
Recently it has looked as though my 5" comet has been doing the mating dance with my largest, constantly rubbing and swimming around persistently, and today I noticed that the fish has a small black area on the tail.
The fish is almost entirely white, with just a little bit of red in the tail, and a dot of red on its head, so the black is very easy to notice, so I assume this happened within the last 24 hours.
I don't see much online other than possibly ammonia burn or finrot, but with the low ammonia, the lack of any damage to the fins, and the fact that the three other fish seem to be just fine I'm at a loss.
Does it seem possible that in the mating dances he somehow irritated his tail, or became stressed, or something else?
Thanks for any help, I'll attach a picture when I get a decent one.