Pam, sounds like maybe the water is finding another path under the tub. Is the joint between the wall and tub well caulked? How is the bath surround constructed? You can use caulk with a mildewcide, but the mildew is really a symptom of moisture where there should be none, especially if it is behind the caulk. If you have access to the underside of the bathroom floor eg. via crawlspace, check it for signs of dampness. PS. Most people don't run the fan nearly long enough; I doubt this is the problem, but a humidistat control would eliminate the possiblity.