A couple of weeks after 1 newly introduced neon died, I spotted another with a white 'saddle' below the dorsal fin in my tank . I got a hospital tank and quarantined the neon, and after research realised that the difference between Neon Tetra Disease and false NTD was that the latter was treatable. I hoped for the best, and dosed with 'anti internal bacteria'. The neon, after 8 days, has its blue stripes back I am not going to rush anything, and am keeping a close eye on the other 4 neons and 5 glowlights in the biorb, but wondered if anyone has any experience of this, and when, if ever, it would be safe to re-introduce my poor little guinea pig back to the group?

No. Personally, I don't think that when a fish has NTD it looks like anything else. It is very clear in it's symptoms. Many people don't realize that NTD is not a Neon Tetra specific disease. Any fish can get it and it was only called that because it was first discovered on a Neon Tetra. The point is, that when people see something ailing a Neon, it is too easy to jump on NTD as the first possible thing.

I did research, and the white 'saddle' was mentioned as a symptom of both NTD and False NTD. My real question is, as I assume it WAS NOT NTD as the fish seems to be recovering, at what point is it safe to return it to the tank?