Hi.
I don't have any small ducks, but for
 the ones I have, I use -
 among other things - 
a resin and fiberglass pool specifically made for waterfowl.
Here it is - filled with exactly 400 liters of water :
View attachment 3976465
Being covered with fiberglass, this pool is really easy to clean : indeed, the surface is so 
smooth wiping it with a sponge is largely sufficient to clean it... EVEN if you 
don't use any product/soap!
I 
do use dish soap to wash it, but just because I am 
kind of a clean freak and just like to do more than needed (instead of not enough)...
View attachment 3976467
(...And that is pretty to look at, right?)
Do they sell these kind of pool, in America...?
Where I have bought mine from - in France -, they don't make them anymore... probably because of
 inflation... 
(too expensive to produce)
But if they make them in America, or if you know of somebody who would be able to make them, then I would really recommend you to get one of these man-made, fiberglass duck ponds...
...The polyethylene ones?
...that are made for 
fishes...?
Do they last for years? (More than 3, I mean?)
I really wanted to buy one or two for my ducks, but the seller told me they were "not made for waterfowl"...
Are they thick enough to 
not be clawed by ducks (and geese?), in 
your opinion?
I would like to buy at least another pool for my ducks, but I don't find any information about using polyethylene pools for waterfowl... (and they don't make the fiberglass ones anymore)