How do I keep humidity trays clean?

[Country Gramma]

Hi All,

I'm sure I saw some info on this in browsing other threads when I was learning how to set up for indoor growing under lights but I can't find the info now.

My humidity trays need to be washed out completely every week. The water in them gets an oily??? looking film on top of it and just looks generaly yucky.

Can I add Physan or something else to it to keep it cleaner?? And if so in what amounts per gallon should I mix it?

Thanks, Karen