What is this black colored fin on my Glofish Tetra?

[Grandevic]

I have this black colored fin on my Glofish tetra. It started out as a little black dot on the fin, but now it has covered the whole fin. What is this? Any help would be much appreciated. Thanks in advanced.

 

[Delapool]

I'm wondering if this could be some natural colour (although unusual). Does the fish look otherwise healthy?

Other options are localised tumour causing colouring change or chemicals eg ammonia spike.

 

[Grandevic]

The fish is eating and doing regular fish type stuff. If it was a ammonia spike wouldn't the other fish be in trouble as well.

 

[Andy Sager]

That is the adipose fin of the fish. Considering the fish is a color morph of the Black Tetra (http://animal-world.com/encyclo/fresh/characins/images/BlackSkirtTetraWFCh_C2975.jpg) you can see in that pic that it's a natural color for the unchanged fish. My guess is that it's just a throwback to the original coloration of the genetically original fish.

 

[Grandevic]

Okay very interesting.