Moisture Trays????

[Faz]

As a new person to orchids, and growing inside in the winter, I use trays filled with stones and add water to keep moisture around my orchids. I have noticed that the trays are collecting algae at the bottom of them. What do you guys do other than removing all the stones and washing out the tray and stones? . Is there an additive I can use or is it the old remove and wash routine?
Also what do you guys use as moisture trays?

Thanks for any suggestions......Faz.