Planted Tank Enthusiast
Join Date: Feb 2013
Location: Jacksonville, Florida
0 Nitrate? Odd. You should have something in there.
Red around the gills sounds like ammonia burning. However, you have other fish living... Hmm.
I don't think it's NTD, but if it is, you should just cull the entire lot of remaining neons. If you're getting your stock from the same source, try a new one.