I had mine hatch out quite a few a while back (probably still are, but I haven't noticed recently). What gave it away for us was a pretty loud splashing noise at the top of the water, followed by our pajama cardinal going crazy zooming around the tank. It seemed to always happen within about 15 minutes of midnight. The clockwork was amazing. We tried to raise them a couple of times, but we always had problems either feeding them enough or keeping the tank water clean enough (can't run filtration and it's really hard to clean out the water without killing a bunch of them). If you want to try the next time it happens, shine a flashlight in (they'll go to the beam) and run a pretty small hose with a really slow siphon into a containter. Each time we tried we got probably 100 or so into our breeding tank that made it at least a week. The feeding/cleaning issue got us from there, but we were able to get a few through about 2 months. By then, they were still tiny, but were starting to get their color and definitely started to look like adult shrimp. It was a lot of fun to give it a try, but a lot of work.
Then again, even if you don't want to try, they do make some good nutritious snacks for your fish... so you can't go wrong.
Your reef will probably enjoy the snack of shrimp larvae...
