Tank size: 50 gallon

Ammonia: 0

NitrIte: 0

NitrAte: between 20 and 40 (some of the numbers are a range based on my comparison of the colors on a test strip to the colors on a chart)

PH/GH/KH: ph - 6.8 to 7.0; kh- between 80 and 120

Cycled,yes or no: yes

Number of fish: 3 long-finned black skirt tetras, 6 bloodfin tetras, 6 zebra danios, 8 adult corys plus 2 juvenile corys

Plants: Live java ferns

Acclimation process: float bag of new fish in water with top open, slowly add tank water to water in bag, net fish into tank

Physical signs of illness(IE spots,shimmer,ECT): bump on dorsal fin (see description below)

How often between fish additions: not often, but added 2 of the adult corys a few weeks ago

Waterchange schedule: 20 percent water change every few weeks

Tank temp: about 75 F



My oldest long-finned black-skirt tetra (a few years old) had a white bump on the top edge of its dorsal fin. From pictures online, I thought it could be ich. I treated the tank with methylene blue just in case. The bump went away. This also happend a year or two ago, and I treated with meth blue then, also, thinking it might be ich. (at the time I had more than one black-skirt tetra of this size, so not sure it was the same fish, but it was in the same place.) No other signs of ich on this or any other fish.
Then a few days ago (a couple of weeks after treating) the white bump was back in the same spot. Now today I noticed it no longer has a white bump, but there is a black bump. (Not sure if the white bump turned black, or if it went away and a black bump grew.)
All the bumps are about the size of a large grain of kosher salt. I don't think it's ich, because it wouldn't always be one bump exactly on the top edge of the dorsal fin on the same fish.
Also, sometimes I think this fish seems kind of lethargic, but then it will act like it's fine, so that part may be my imagination.
Also, when it had the bump a few weeks ago, it might have had a second smaller white bump also on the top edge of the dorsal fin, but I can't remember for sure if it had one or two; now it is definitely just the one black bump.
This fish has a very long dorsal fin, so I wonder if it could be an injury to the dorsal fin rather than an illness. Any thoughts?

the white bump sounds like ich to me. Anyway here is what I found about the black bump.

Black Spot or Tang Disease - Black Ich Diagnosis and Treatment

I don't know if it is true or not just what I found. All the symptoms you described seem to be mentioned in the article as well.

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