Water Butt

I have had a plastic water barrel (with lid) that I used to fill from rain running into a container at the front porch. It was always clear and fresh.
A couple of months ago I fixed a diverter to the main downpipe of the house, the water in the barrel is now starting to smell, Why?, anyone know and what should I do about it please.

 

So sorry, just read another on the same subject, like they say if all else fails read the instructions. :D
I too will try charcoal I think, and maybe the tights idea. I would hate to have to do without.