Black Neon Tetras

[chesterjohn]

Not entirely sure if this is an emergency, but I have a black neon tetra that has gone red on it's belly and in the face. There are a couple of others that have started to go red underneath too. Is this likely to be anything bad?

They still eat like normal.

Water stats are:
ammonia - 0
nitrite - 0
nitrates - 20ish

ph is the same as normal

 

[Curiosity101]

Could you get pictures? They would probably help quite a bit.