I clean my tubs with chlorhexidine whenever they use the bathroom (If I have a lack of time I spot clean the area and replace the paper towl but normaly I clean the whole tub with the chlorhexidine). The most use of the bathroom I will allow is two in their tubs. This happens due to lack of time on my part. If two builds up in there then I find the time to clean it. But I only have like a dozen (and a bit of ocd). I do a complete cleaning every few months with a bleach mix.