Ohh. Read that wrong. I'm not sure how fast these guys grow up. But they seem to be decent size if that matters. Would say close to 3/8 of inch. Some a bit smaller. Smallest would say 1/4. Is size good way to judge age?
I would agree with Odin and try to shut off the filter even though it is very low. The opae can withstand very harsh conditions and don't require any filtration. Water will be crystal clear without it. If there's places to hide the shrimp will. I with 100 opae for my nano cube 6 gallon. Saw them for the first day then almost all of them retreated to the rear of the tank that's for the filtration. I do not make use of the filter. I've taken it apart and removed the walls that space it out and have also opened up the hole so the shrimp have an easier time retreating to the rear. This is the biggest regret I have in having a tank like this. Unless I blocked off the back they will always go back there. I would estimate that 90% are back there at any given time. Same thing for my nano cube 3 gallon. Here's a pic of my nano 6 when the opae are hiding. It has more than 200+ in it.
I think you're talking about the hole that leads to the rear. There's both lava rock and coral pieces back there. You might be talking about the white coral in the left. Haven't touched the tank too much. Just letting the macroalgae grow. Tank is doing well so I really don't want to mess with it for now. If I feed they all come out.