Firefish group always ends up with 1 left

[Miami Reef]

I’ve had my reef tank for a few years. I’ve always tried to have a group of firefish to school together but always one would be left. I never QTed before.

Do firefish do well in groups?

 

[Grsfish]

I started with 4 on my 210 gallon and went down to 2 pretty quick and now I have had the same 2 for about a year