You mentioned that you went to Colorado. I do know that there is a well-known holistic Vet in Boulder, CO. Unfortunately, I don't know his name but you may find him if you did a Google search.
"There is no visible mold, but I have to keep the tub clean or it builds up around the drain."
Hmmmm.... You might want to dig into that deeper. You may have an "unseen" mold problem with it appearing around the drain. Sometimes the biggest mold problems are the "unseen" and hidden ones; behind walls, in furnaces, etc.