Salt water. Salt water should be made from high quality marine salt, table, rock salt etc should never be used. We use red salt mixed with RO (reveres osmoses water). RO Water is cheep however it's only commonly sold in 10ltr & 25ltrs, marine salt is expensive, and the smallest amount you can buy would last you years! We now sell  enough red-sea salt for 2ltrs, just add to 2ltrs of mineral water for great marine water to give to your hermit crabs. Price £1.50