This is a newly setup tank. It took around 7 weeks to completely cycle. It is a 42 Gallon Tank. I cycled it with 8 Long Fin Danios, they did great and have been happy the whole time. On Sunday The water peramitters were the following so I decided to increase the fish load.
Nitrate- 10 ppm
So I went to the lfs, to pick up 3 oto cats, and maybe a couple of shrimp.
I had a decent amount of store credit and after looking around and a little pushing from a worker I left with 17 fish, and 5 shrimp. Below are the critters I picked up.
I realize that this is kinda a lot of fish to add at a time to a freshly cycled tank, but I guess the excitement overcame me. This is my first tank in 10-15 years.
After 24 hours of the new guests inhibitting the tank the water peramitters have remain the same with the exception of the Nitrate levels rose from 10 ppm to 20 ppm.
Now to what this post is about, I was inspecting the fish tonight to watch their behaviors and look for any signs of stress. They looked excellent and really have settled in rather quickly.
However one of the cardinals had a very rather distrubing pimple up right underneth his dorsal fin. At first I thought it was Ich, but sence have kinda thought is was rather large to be ich. There is only the one pimple, when you look at it from head on, you can really appriciate how large it is, it definately protrudes outward from the skin. I havfe seperated this fish from the tank, and plan to take it back in. I am attaching 3 pictures. I know the first two are of very poor quality. Trust me those are the best of about 50 pics taken with 2 different cameras. The third is a pic from the net that I edited to give a clearer idea of the problem.
I am most worried about it spreading. And want to start treatment as quick as possible if that is indeed the right thing to do.
First off, as you've already said that's a LOT of livestock to add at once; there's a real possibility of crashing your biofilter...if you can, you should get some biomedia from an established tank and/or dump a bunch of Seachem Stability (which is basically shelf-stable bacteria), which should potentially help. This is really your bigger problem since if your filter crashes your tank crashes.
Now, with your tetra, isolate and treat. If someone on this board can't tell you what it is, and I can't because the pics aren't ideal (it would be difficult to get ideal pics for this w/o a macro lens) I would suggest taking him to your LFS and asking them, if its close by.
A somewhat homeopathic-type treatment that people swear by is MelaFix (and Pimafix); supposedly these cure a lot of issues for people though I haven't personally tried it. This would be a broad spectrum solution, diagnosing it as parasitic / just a bump-into-glass injury / bacterial would give us more info and let us better treat it.