What happens when you have a tank full of shell-dwellers and you go to get some out, and... they're GONE?! So you remove rocks, shells, and sponge filter... and still... no fish. Where are they? Well, being shell-dwellers, they are IN the shells (and crevices in the rocks) and no amount of coaxing is going to get them to come out.
A breeder of Neolamprologus multifasciatus in Australia told me once that if you get the shells up high, the fish will come out and drop down, since they are bottom fish. They won't go back UP to get in the shells... so here's what I did:
Into a tall rubbermaid tub (a big bucket would work too), I siphoned off tank water, as if I were doing a regular water change. I put a shallow flowerpot in the tub, with the opening up, and then I placed a cake rack on the flowerpot. Anything with openings large enough for the fish to pass through, yet small enough that the shells do not, will work just fine. I then added enough fresh water to cover any shells that I would place on the rack.
I placed the shells on the rack with their OPENINGS DOWN (otherwise the fish just hang out in the opening), along with any rocks that I pulled out of the tank, as the fish will hide in these also. I left this alone for about 30 minutes, and when I came back, I had about 40 fish swimming in the bottom of the tub, some of them conveniently located inside the flowerpot.
I wasn't worried about getting every single fish, or I would have waited much longer. So I put the rocks and shells back in the tank, and then sorted through the fish, pulling out my breeders and any very small ones and returning them to the tank.
I've used this method several times, and it works. The really cool part is that the fish don't catch on and foil you after a while :-)