Grass shrimp are actually full sized at about an inch and a quarter. What a great idea Gary! They carry eggs too and have live babies, so putting a wad of them in you sump will produce live zoo plankton every week. That would be good any time, actually. They'll thrive on a balanced pellet food or chunks of stuff you feed your fish too. The same is true about peppermint shrimp too, if your sump is large enough. Those guys have hundreds of babies twice a month when things are right.