Some ceramic stones don't mix with soaps or detergents, particularly Shapton Pro's. Don't know about Naniwa but consult the instructions to be sure.
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It says in the instructions that some laundry detergents are okay but liquid dish soap is not. I guess when it dries it retreats into the pores as a concentrate. Once wet again it expands to 4 times it's size which pulverizes the stone from within. The result is about 1/4 " of the surface basically turning to sand or fine powder. As long as the stone never dries you will be fine. Which proves once again you don't have to read the instructions because everybody has a well prepared reservoir to keep their hones in right ? Sorry, sometimes I can be an evil joker :<0)