I've read that many people have great results with this
http://www.mops.ca/cgi-bin/SoftCart....fd5+1089066264
you should also try to figure out where/why this is starting. Are you using r/o water? how old is the unit if so?
IMO you need to pinpoint where to problem is and solve that because no matter how much you dose if you keep re-infecting the tank you'll keep having the problem.