If you can, take it apart & clean it. You may be able to add a tiny amount of bleach to the water. The scum may be algae; there are products you can add to control it. Have you looked in the instruction booklet? You might try asking at the water garden area of a nursery or Lowe's, or such.

flush it, put some of that liquid that they use for water beds in there and let it soak for a while or use swimming pool clearifyer on it.

Well I would do a number of things to this.........

As long as it has no fish or plants in it, I would take it apart and clean it throughly, I would use maybe Bleach.........if it is Hard Water.......I would try maybe leaving it set in CLR..........or maybe I would try Lime Away.........

There is many things that you can do to get it cleaned.........I would not use the same water in it again.........I would buy distilled water for it.....that way it would be much cleaner the next time.........

Hope this works..........

I have had the same problem. I have had to take ot all appart as previous posters suggested, clean it up with Lime Away or CLR (both work fine) and put distilled water with a touch of bleach in it (not much), it keeps the scum form returning. YOu will still have to do this periodically but not so much if you use distilled water with bleach.