Isn't that how antibiotics and insecticides lost their effectiveness?

1 teaspoon/gallon is about half the strength of a normal dose of Physan, and
I'd think that once a month wouldn't be too much, but maybe the resident
chemists will chime in here. Also, one could vary the product. Maybe Physan
one month, Phyton the next, and so on. Just my 2 cents.

Diana