My neon tetras look fine but have a bright red area at their gills. Any idea what it is?

They still have their strong characteristic colouration, and i dont observe any lesions or growth on them. I keep them with no other fish in my tank, though admittedly, im still trying to cycle my tank (since early February). These red colourations only appeared recently. It doesnt sound like any of the diseases ive read up - or maybe it's in the early stages? Someone please give me some insights!!! Thanks