If we are talking about porcelain crabs ~ At one time I had one in my 55 gallon tank about 5 years ago. He would sit on the rocks and I used to feed him zooplankton, he would extend his third maxillipeds (those little catchers mits) and catch those little plankton bits and chow down. If I were to ever try another one I'm sure that I would also include cyclopeeze in the diet.
Thomas