What's the matter with my green neon tetra?

[DaPrawf]

The smallest of my green neon tetras has recently developed an odd coloration. There's what seems like rusty brown behind his head. I've tried to catch him to quarantine but it is a heavily planted tank and he always finds hiding spots. His swimming is mostly normal, although he seems to swim in spurts. Oddly, about the time I noticed his coloration, the whole school of green neon tetras became quite skittish. When I approach the tank, they run and hide. They eventually come out but they're skittish. I haven't noticed any discoloration on any of the others. I have about 12 green neon tetras, one honey gourami, three guppies, 6 orange corydoras, 4 amino shrimp, and maybe 6 red cherry shrimp. Everybody else seems fine.

29 gallon tank

Ammonia: 0

Nitrite: 0

Nitrate: 15 ppm

pH: 7.2

dGH: 8

 

[Colu]

Looks like a bacterial infection I would do a course of kanaplex in food feeding a small amount twice a day for 7 days and add a small amount of aquarium salt 1 table for 5 gallon the salt will aid Gill function and add essential electrolytes as well and at that level it shouldn't harm your plants