Stagnant water in garden feature

[Andy165]

Have got a garden feature of a large half barrel with another smaller half barrel on top , an ornamental hand pump on top linked to a basic pump in the bottom barrel to circulate water running from the top to the bottom. Problem is the water is getting stagnant and smells a lot.
Is there something I could add to the water to "clean it up". Or should I put the pump on a timer to circulate for half hour a day to aerate the water to try to keep it fresh.
Went to local garden centre who also suplly fish and products but they could not recommend anything. Is there a tablet of some sort that I could add to freshen it up but not harm the local wildlife that may drink from the water feature?
Any ideas appreciated.

 

[merlin50]

change the water, simples

 

[xdave]

If water isn't aerated then it will become stagnant. Merlin is right - you need to change the water and then ensure that the pump is activated for a few hours a day (more on hot days as warm water holds less oxygen). The movement will encourage gas exchange and prevent it becoming a stagnant death trap. Maybe have a few aerating plants too as they will encourage all sorts of wildlife to use it. Plants will help to prevent algae too if you get the balance right.

You don't need to worry about any wildlife in the barrels at present (it is likely to only be some very late mosquito larvae and bacteria). I would never add any chemicals to water that is used as a wildlife pond - isn't that defeating the purpose?