Well, here's what I can tell you...I can't guarantee the accuracy of ANY of this, it's just my observations and memory.
We have a compatible pair of the standard Red Firefish. I purchased this pair by watching a group of adult fish in the LFS tank. Let me say now there is probably no really reliable means of sexing these fish. Let me also say that I'm really careful to describe these fish as a compatible pair..they've not spawned for me that I'm aware of..they very well could be 2 of the same sex.
Anyway, I watched a group of 5 mature firefish at the LFS for about an hour. 1 was being torn to shreds in the 40 gallon long. Once I identified the aggressor, I presumed I had found the male. I kept my eye on this aggressive fish who would periodically go and attack the other fish, quivering in the corner, literally shredded.
About 45 minutes into it, I noticed this dominant fish (our likely male) swim in front of another firefish and, while perpedicular, flar his fins and kinda shake a bit...he then wenton his merry way. AHA..the dominant fish had pointed out one of the girls in the tank.
I had to try really hard to keep the identifying characteristics of each individual fish in my head while the shop owner pulled out the specific fish. They were added to our 24 gallon cube at the same time and pretty much shared their bolt hole from the get-go. One is definitely fatter, larger and more outgoing..the other is smaller and spends most all of his time in the hole, only emerging to feed. If I had to say which was which, I'd guess that the male is staying in the bolt hole and the female is the larger, dominant and fatter fish who comes out more (but still hides a fair amount too).
I've heard 2 scenarios that possibly could be how firefish "do it". The first...they supposedly will spawn in the bolt hole with the male tending eggs. If that's the case, mine could be spawning all the time..I just haven't seen anything come of it.
The other thing I've heard is that Firefish actually spawn in a pit dug in the sand outside their bolt hole. Haven't seen any pits dug in my tank.
That's what I can tell you. All the information points to the fact that in a tank our size, if we did not have a mated pair one should've killed the other by now. I just don't have any "proof" but maybe someday I'll give them a dedicated breeding tank decorated with PVC and sand and an overturned tridacna shell or something...anything to make it easy to observe and see what's going on.
FWIW,
MP