Can I use kosher salt instead of sea salt for hermit ?

you should not use kosher salt in either a fish tank or for hermit ... kosher salt has an anti-caking agent added to the salt which would be detrimental to both fish and hermit ... for your fish you require aquarium salt (not marine salt) and for the hermit you require marine salt (not aquarium salt) ... be sure to dechlorinate the water before you mix up the salt water ... also, do not buy any special hermit crab salt such as zoo med parts 1 and 2 ... i made salt water with their mixture and absolutely no salinity was detected when i checked the water with a hydrometer ... part 1 is questionable too ... a fish water dechlorinator is what needs to be used :O)