Rain water from the roof would be somewhat tricky, although not 100% impossible, because of the distance from the house to the duck kingdom.
However we do have a well, and it's about to become solar powered in the new season.
As to big, yes I understand the need for it but I probably don't have the room to go much over 4 x 4 or 5 x 5 m which is still very little for 16 ducks (half a cubic meter per duck, they can mess that up just to start their day) -- so yes I'm ready to replace the water often.
Since the water would be reincarnated as irrigation this is actually not entirely bad. I'm even thinking about a continuous slow inflow-outflow.
Do you have any strong feelings about duck weed? Not using weed for ducks (also an option I guess:
https://www.bbc.com/news/world-europe-11799303 ) but rather this stuff
https://www.rhs.org.uk/weeds/duckweed .