I picked up some juvenile yellow labs from a friend and didn't think much of them (about 15 of them in a 20g) until one day I noticed, not one but 2 of the females holding. They didn't even appear mature enough to be holding. Anyways, since then, the juvies have all since gotten bigger and there were just way too many labs for the 20g ... so I started taking the bigger labs out and putting them in my 55g. I'm sure you can guess ... the larger fish were almost all males ... so I ended up with all females in the 20g (except the one nice looking male that I left in). I typically have 3 females holding ... usually all at the same time (each holding around 10 fry). Then they take a week or 2 off and then all 3 are holding again. The weird thing is ... I'm fairly certain the other 5 or 8 fish in there are also females, but they never hold.
Anyways, I've had all kinds of fun trying to figure out whether stripping the females or letting them hold til they spit is better.