I suppose the water smells because it gets stagnant. I presume you only use the water in the barrels to water the garden with and not drink...I don´t think that the plants etc will mind if it smells a bit.

So far i dont fiond either of mine smell but I use them as almost as soon as they are filled.

Thanks Rogi, I use this particular barrel to water my conservatory plants ( I have a lot ) more like greenhouse really.

Have you checked there is nothing dead in it?

Always keep water barrels covered with old lengths of wood. Stops birds thinking its to drink and drowning.
Keeps water fresh for months.

If it starts to smell you can add a splash of hydrogen peroxide which will kill the organisms that cause this, just like pond oxygenators but quicker. Let it stand for 24 hours before use. You could add a splash of standard chlorine based household bleach but would then have to let it stand for a couple of months, by which time it would be ponging again. In other words, don't.

Either use it or regularly run some of it off. When there is a dry period......if there is ever one again......take the barrel down and hose it out, if you can.

Thanks folks for your advice I must add that it is a small barrel just inside my garage and I suppose I should run it off more often and give it a clean out.I don't think there is anything dead as it is covered but who knows ? Will have a look.