Black Neon Tetra, injury or disease?

Hi everyone I would really appreciate some advice and input on one of my Black Neon Tetra.

Some background info on the day he got affected and what his current behavior and appearance is: I was doing a water change last Friday (I do two 40% changes a week one on Fridays and one on Tuesdays) they usually get a bit skittish when I trim plants and clean the glass in the tank and go quite pale but then I noticed one of them, instead of losing his colour, went completely black on his tail end. Almost like all the colour from his stripe drained into the back of his body! He was also swimming in a slightly shaky movement. I took a photo and tried to google but couldn't find anything. Later that night his colour returned to normal and he seemed fine.
Cut to this morning when I switched the lights on I noticed him doing the same strange swimming motion only this time his back end was very white and the black stripe part looked a bit splotchy. He didn't eat breakfast this morning and I am very worried about him!

Sorry about the potato quality pics it's the best I could do. I'll follow up on any info I might have forgotten if anyone has any questions, much appreciated!

 

Your description sounds like chemical damage, either chlorine, nitrite or ammonia.

 

Is at always the same fish, or all of them?

I find it odd that its only 1 fish getting it if its water parameters, unless its new and was raised in different parameters.

@Adanya what is your tank volume?

I would recommend you add some rooibos tea in to the tank, when I had a big tetra tank that always seemed to help them along when things got difficult.

 

Thanks for the reply, after I noticed the issue I did test the water, nitrite and ammonia were both zero and I dechlorinate the water when I change it. It just affected the one tetra in the picture and I have 9 others and they all seem fine so I'm not sure it's a water parameter issue.

It's a 125l tank and all the tetra came from the same source and have been in the same tank together for months and like I said in my reply to BoelderBeestie it's just the one little tetra that's affected the others and the rest of the fish in the tank are all healthy and happy so I'm stumped. I will give the rooibos idea a chance to try and help the little guy out thanks for the idea and the reply.

 

Does the color change only happen after a water change?

 

Not sure if I haven't been observant and only noticed it today or if it only happened again today/ last night.

I use Seachem Prime, which states on the bottle that it removes both chlorine and chloramines.

I really appreciate all the input everyone. Thanks again for taking the time to reply!

 

My Harlequin Rosebaras used to undergo massive color changes after a big water change and then I would notice them dropping eggs...

With only 1 fish showing this I don't think its water parameters, but something else...

 

Thought it was all of them but re-read it and see it's only the one, you might be on to something here oupoot. The shaking movements and all.

 

Lets hope its breeding behavior, but either way the rooibos was also a treat that seemed to make the fish happier