Unusual yellow head found in dead fish.

[Liddojunior]

I got a couple new fish that I ordered online. Ordered chili rasboras and CPDs. On arrival I had half the chili rasboras dead.  I put them in a cycled QT tank. The chili rasboras all died, fortunately the CPDs are looking good. They have colored back up and eating well. But had a couple suddenly die with distinct yellow internal coloring. 

Picture attached shows a CPD that is clearly colored up, the fins if in water are fully visibly orange. But the belly and top of the head are very visibly bright neon yellow. As all the fish in the QT, not concerned about spreading but would like to have an idea of what it is so I could treat it effectively. 

Tank parameters are Amon and Nitrite 0 ppm. Nitrate 10-20 ppm. Hard water. pH 7.2. 

Edited March 17, 2023 by Liddojunior
Added image

[Colu]

Yellow colouring possible velvet or a bacterial infection if it s a bacterial infection  what I would do is add some aquarium salt 1 table spoon for 3 gallons and do a course of  maracyn2 or kanaplex if it were velvet the most effective treatment is Fritz copper safe you will need a copper test kit to keep the level of copper at a therapeutic level of 2ppm  he's a picture of velvet for comparison  @Liddojunior