Can I suggest a little solar fountain? The ones they sell for bird baths are ideal and add the sound of moving water which is lovely and also seems to attract more wildlife.
@FlyDragon I find they do help and they certainly oxygenate it. I have a little tub of water for the bees on the allotment and the fountain bubbling away seems to attract them. Some people say they keep mosquitoes away not sure how true that is.