I presume its got a lid or I would suggest that something dead might be down at the bottom?

If the top is open on your water butt, leaves etc. have fallen into the water and are decaying at the bottom. Use the water from your butt on large shrubs, clean the butt thoroughly and replace it with a lid on top.

Our rain butt is open and himself even soaks the clay pots in it to get off the worst of the soil... No smells at all! Personally I would not recommend covering a rain butt.

The smell may have been picked up from something in the atmosphere, we get oil on the top of ours from the Heathrow flightpath. Rainwater has a smell of it's own when kept in tanks or butts.

Perhaps as MG said leave off the cover HB?

If you leave the cover off you're likely to have mosquitos breeding in it--not a very nice thing to happen. Perhaps just cover it with very fine mesh or fleece to stop the little blighters from getting in. Our covered butts smell as well, but it doesn't seem to have any bad effect when I use the water. Best thing to do is to recycle the water quickly, so that it refills frequently whenever it rains---not a problem lately!! Annie

Anne we've never covered our rain butt and we've never had mosquitos breeding in it. Drowned slugs and snails yes, mossies nope.

I have 4 waterbutts and I find the water in the one that is furthest from the garden and therefore used less---usually when the others have been emptied--does smell nasty. It is perfectly ok for watering though, so I just brace myself and fill the can! I can only think it's because the water in that butt stands longer and gets stale. I always keep my butts covered because I once found a blackbird drowned in an open butt, which was rather upsetting.