Question
Do ducks like algae? do they eat it? can it harm them? Have a 30 foot by 100 foot pond with plastic membrane at bottom and sandfill __ is that appropriate? Should sand be replaced with soil? depth of pond is two and half feet and it is kidney shaped.

Answer
Ducks do like algae.  They eat it as well as other vegetation.  By 'sandfill,' I assume you mean under the liner and not in the pond with the water.  Sand is, in my opinion, the best material you can use for underneath and around the sides of the liner.