In today's OGD, Bill Bergstrom mentions a dilution of two to four drops of Physan per gallon. That is probably useful for mixing small quantities, but not easily generalized to larger quantities (I don't know how many drops there are to a teaspoon, or a milliliter). My own experience is that a ratio of between 1/2 and 1 part per thousand works fine; it suppresses algae and has no noticeable ill effect on the orchids. On occasion I have inadvertently used dilutions of maybe 2 parts per thousand, again with no noticeable ill effects on the plants.
Giles Smith
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