humidity trays developing scum or film

[mykelde]

I purchased a humidity tray for some of my orchids but it seems the water that collects in the tray starts to develop a film or scum ( bacteria ? ) within a few days. I tried adding vinegar and then tried bleach but that doesn't seem to work. Maybe I'm adding too little - I'm afraid of the roots touching the water with vinegar or bleach in it. Besides replacing the water every other day + washing it is there anything I can do so I don't have to do this ? Thanx for any advice.