I'm thinking of putting a small pond in for my ducks, something like 150 - 200 gallons. I don't want to use the type of liner that comes in sheets and you roll it out (fabric liner I think it's called?) because I would be worried my ducks feet could puncture it. Because of that I was looking into the preformed ponds that are ridged plastic.
I would use either 40 mil butyl rubber liner or a Rubbermaid horse trough. Both are cheaper than the flex pond and ducks will not harm them. Just remember that ducks are messy and you will be cleaning the pond at least once weekly if not more often. That means pump out all the water and wet vac the bottom. I would use a kiddy play pond that you find at any big box store for about 15 dollars.
We installed a butyl liner in a much larger pond than yours, about 15 years ago, and it's still going strong. Butyl does come in different grades, and it would be best to get a heavyweight one, but you won't need to worry about the ducks puncturing it, it's ever so strong. Preformed ponds do limit you to their shape, it's quite hard to get the hole exactly right to support the form, whereas a butyl liner just takes the shape of the hole as the water presses it down. Calculate the depth X 2, plus the width of the pond, in each direction, and add about 3 feet to each dimension, so you'll have plenty to overlap the edge. You could then cover the edge with stone slabs, to preserve the edge from getting muddy and worn away by the ducks as they go in and out of the pond. Spread a 2'' layer of sand over the sides and bottom of the hole before laying the liner, to cushion it against any stones etc left protruding from the earth. 150 -200 gallons isn't very big for ducks, and will quickly become polluted. If you are able to, a bigger one would be better.
