OK, I found something on the web:
QUOTE:
In that sharpening article, it recommended submerging waterstones continuously when not in use to keep them from disintegrating. That’s why I put a tank in the bench. I used to use waterbed additive to keep the mold and algae at bay and to prevent the water from getting slimy. I ran out of this stuff, and I can’t find it locally anymore. Do you have any suggestions for a similar product that won’t hurt my Norton waterstones?” – John E. Adams
Tim Inman: First, I would strongly recommend you follow the stone maker’s recommendations. If they want the stone constantly wet, then there is your
answer. If you use the stone often, I would keep it wet and ready. But, if you use the stone only occasionally, then it might not be so important to keep it constantly wet. It will take a little while for it to saturate correctly before you can use it properly again, though. Waterbed algaecides are commonly available. If this has worked for you in the past, then I would continue. “Blue Magic” is one brand I have used, and it is available all over the Internet.
Brian Leonard: Different waterstones will have slightly different compositions, which might react differently to additives. Try adding a little bleach to the water and keep the tank covered. This
should prevent the water from getting moldy or slimy. Be sure to check your waterstones periodically.
(from woodworkersjournal.com).
Unfortunatley, the directions for this stone are in Chinese, so I don't know if it's supposed to stay wet. And I don't want to use algaecides or anything, but it seems changing the water would be necessary.