Humidifiers and Dehumidifiers - Humidifer Gets Slimy

I have a Hamilton by-pass drum humidifier I installed on my furnace last year, and it worked great the whole season. In the spring I drained and cleaned it. This fall about 2 weeks after filling it, I checked it since it seemed like the house was too dry. The sponge wheel and tray was full of a nasty goo that smelled musty. I took it all apart and cleaned it with bleach and replaced the foam element. Once again in about 2 weeks it was full of goo again. I cant understand what is different from last year that would cause this problem. The damper is open and I get plenty of airflow through the unit since a full pan of water will evaporate in 24 hours. I realize this isn't the cadillac of humidifiers, but its what I have to work with right now. I'm hoping someone can help me out here.

Are you on city or well water?

Soft or hard water?

Than would be city water, right from a tap on the cold water line feeding the hot water tank. I couldn't tell you if it soft or hard, I have never had a problem with faucets liming up or any staining/deposits in sinks or toilets.

You own the home or rent a condo?

A water softener is a unit that you put salt in, if you don't have that, then it may be very well be the type of water you have in your area.

I think i've seen tablets that you can buy and put in the pan to treat the water.

I own my home, guess I'm just hung up on why it's a problem this season and not last year. Like I said I have not seen any excessive liming issues anywhere else in the house. I just don't want to have to tear it down and clean it every 3 weeks. I have some conditioner I can put a capful of in there every few days, but i'll look for the tabs.

I'm not sure who sells them. but if you Google "humidifier tablets" you'll see a few of them on line that you can buy it from.