Land hermit crabs require two water sources, one fresh and one ocean (salt) water. All water must be treated with a product like Prime to make it safe for the hermit crabs. Water pools should be deep enough for your largest crab to submerge itself. Deep pools like mine can have filters to extend freshness of water. Be sure to include a way for the crabs to climb back out of the water. Some species seem to really enjoy swimming and maybe some day I can catch one of mine in the act!