Disease on Rummy Nose tetra's head?

[Taiwanfish]

Hello all,

I am looking for some advice.  So here is the brief story: I bought 4 Rummy Nose Tetras from my local Petco few weeks back. They were the last 4 Rummy Nose (I would have bought at least 8 if they had that many) and they have been in Petco for a little over a week. After quarantine, they did great in acclimating into my main planted tank and are thriving.  Wanting to make the total to 10 Rummy nose, I waited for Petco to get another batch of them.  So on 4/2 they received a huge batch. I immediately went to the Petco store and bought 6, acclimated them and placed them in my quarantine tank.  

Status of my quarantine tank: It established (0 ammonia, 0 nitrite and 10ppm nitrate, and 7 pH). No sure what the water hardness is. Water temp is 78. Hang on back Marineland filter.

Immediately 1 of the tetras died that night, 4-5 hours after I placed them into the quarantine tank. The following day, 4/3, when I woke up, I noticed another 2 died.  While removing the 2 dead tetras, I noticed one of the 3 alive tetras swimming oddly. It looked like it was floating in the tank aimlessly, bumping into objects and not being able to stay right side up (swim bladder disease?). Nonetheless, it died shortly thereafter.

So this left me with 2 tetras alive. On 4/4, a fifth died similar to the way the fourth died, floating aimlessly, bumping into objects, not staying right side up. This has been a terrible experience for me, who only been fish keeping for a little over 6 months. I want to do everything I can to keep this last one alive and hopefully he survives to make it into my main tank. 

So my question is: I noticed that on top of his head there seems to be a small patch of green. I looked at various Rummy Nose tetra images on Google to see if this is an abnormal growth/coloring. Some images shows normal silver spot, I don't want to weed out the possibility of a fungal growth on the head, so I want to see what you veterans think. If it's something, how should I treat it?

Thanks for your time and assistance.

KJ

Edited April 6, 2021 by Taiwanfish

[Colu]

With the symptoms you describe it could be ammonia poisoning or parasites