I like my ghost shrimp (I have 4 or 5 of them in my 16g bowfront)... they seem to be very good scavengers, having a VERY good sense of smell. They quickly home in on whatever bits of food have fallen to the bottom. Fun to watch too.
However, they have been edged out as my favorite tank inhabitants by the new additions as of a week or two ago, 3 fiddler crabs. These little guys (actually, they're all females) are CONSTANTLY moving their claws from the gravel to their mouths as they search for anything edible. And they really love this fluffy-looking brown algae stuff that is growing on the fake plants.
I also drop some small crab pellets (for calcium supplement) to the bottom of the tank every now and then. It's hilarious when a crab finds one of these little pellets... the larger crab will usually just try to cram the whole thing into her mouth, while the smaller ones excitedly grab the pellet and quickly scamper off to a cave with their prize to eat in peace.
Another entertaining behavior is when one of them finds one of the larger shrimp pellets I sometimes drop in for the cory cats. I guess these are too cumbersome to carry away, so they pick away at it on the spot. After a while, the fish start to take notice of the crab's find, and a crowd begins to form... as one of the fish tries to move in to grab a bite, the crab's reaction is to stick out one or two legs in the direction of the incoming fish, "stiff-arming" it to keep it away from the food.
I did have to make some accommodations for them... to enable them to easily get out of the water when they want to (without dramatically altering the tank to the detriment of the fish), a small floating platform was constructed out of a feeding ring, with a long single "strand" of a plastic plant serving as a ladder to get there. Though they probably would have found it on their own, I introduced each of them to it by netting them when the opportunity arose, and releasing them on the platform. Since then, I frequently spot them up there, basking under the light. Though they seem to find plenty of stuff in the tank to eat, I leave a few morsels of the crab food up there.
I carefully installed screens across all the openings in the lid to prevent escapes, though so far I have not seen them make any attempt to get out... from what I understand, one of the main factors in these little guys wanting to escape is that they are not provided with a place to get out of the water. I also add a few drops of iodine each week (supposedly this is good for the shrimp too, as it aids in molting).
Anyway, maybe these crabs are not suitable for everyone, but I really get a kick out of watching them work.
--Mike
