A Plumbing Conundrum: Stinky water in toilet and we don't know why!

[Audio_Al]

We have a ten year old home which we built.

Three bathrooms -- three toilets -- all the same style and type and kept scrupulously clean by my wife. Wife and housecleaner scrub everything in that bathroom once a month.

Two toilets are fine -- the third one (probably the least used) has developed a moldy smell when the lid is up.

The smell is coming from the water in the tank and the toilet. Water is clear but stinks like mold.

We used chlorine tabs in the toilets for the first few years, but then abandoned them because of environmental concerns and had been told that chlorine would eat up the metal and plastic inside the tank.

Any other thoughts besides going back to chlorine sanitizing? Thanks for any comments.

Respectfully,

Audio Al