Perhaps they are trying to indicate the water has too much Chlorine in it, Ducks aren't like Chickens, much smarter.Access your water company's website and check Chlorine levels.

You could collect roof rainwater runoff into a butt and use that to top up your Duck's water and only use the tap supply cistern when rainwater isn't available, they may/will likely, prefer it to the constant chemically cleaned tapwater.Peace. : )

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Perhaps the cap is of a size and colour that attracts one or more of your ducks to play with it. I have known ducks to behave quite playfully with water. Sorry to be so vague, but without even a pic of the cap...

Mallards feed by filtering. What looks like a muddy mess to you is probably a choice buffet to your duck.

As for the plastic cap, was there any reason to keep it? Perhaps the ducks are just tidying up. If your part of Ireland is anything like my part of Ireland then tidy ducks would be an environmental godsend. They could start by collecting all the black polythene silage-wrap that litters the countryside.

As I said in the other duck question, birds can take funny notions. If one of your ducks lifted the plastic cap and deposited it in the cistern when he realised it wasn't edible, and then found a tasty morsel immediately afterwards, he might assume that one was linked to the other. You think that's barmy? It's no barmier than touching wood, or greeting a magpie with, "Hello your Lordship, how's her Ladyship?".

Are you sure it's the ducks moving it, and not some other species?