Serpae tetra disease

I have a 55 gallon tank with several various kinds of fish. Recently one of my serpae tetras has a growth/sore where the tail connects to the body. He also has black splotches all over his body. He's more brown then red now. I don't know what disease this is or how to help it. All the tank readings are normal and I do frequent 50% water changes. Any ideas? I don't want it to spread.

 

Welcome to Fishlore!

Can you take a pic?

Can you provide more information? Exact water parameters (pH, temp, ammonia, nitrites, nitrates) and tank mates would be helpful.

How frequently do you do these 50% changes?

 

It's hard to see but there's black/brown patches all over.
I don't know the exact readings but the test strips are all in the good range.
There's a couple zebra danios, some mollies and their fry, a pleco, some amano shrimp and some gold tetras as well as the serpae tetras.

 

Update:

The pictured fish died. I still have one more who looks like this.

 

Test strips are inaccurate, and telling us they're in the good range isn't helpful. I would consider using a liquid test kit like the one made by API. Much cheaper per test too.

How often do you do 50% changes?