First off, unless you're treating him for something, the general recommendation is to not use salt at all. Treatments are different and should only go on for 10 days, otherwise they risk having kidney damage from the increased workload. :)
I've never tried to soften water, but this looks like a pretty complete post about it on the other forum, that might be a good place to start. :) I'd hate to guess and give you totally incorrect information!
If that doesn't help, then we'll track someone down on here who has some experience with it. :)