Water butts in da shade

Organic material can make the water smelly. We had the same problem last year because our hazel had grown over the collection roof and was seeping sticky sap onto the roof. It really made the water stink and it caused a milky colour to form. The butt is covered and has a sponge filter but it doesn't stop fine residue being leached into it.
We have had the same problem with rowan berries in the autumn and dead slugs. I popped the lid off one butt yesterday and it had worms in the bottom as well but it isn't fed from anywhere and has a lid, so they must naturally travel in there themselves.