Here’s a classic pre-formed pond liner.
It’s designed for fish-keeping, and for standing plants in pots on the shelves. It’s not the ideal shape for wildlife.
So how can we make a good wildlife pond out of this?
Simple: do the most important thing you can for any pond – fill it with clean water.
Be obsessive about clean water: it’s very easy to get polllutants into water and very much harder (or impossible) to get them out again.
So no soil, no tapwater, no fertilisers, no compost, no plant food, no fish food, no turves. Clean, washed, childrens sandpit sand should be OK, but absolutely nothing that will add unnaturally high concentrations of nutrients.
Leaves are fine – let them fall in.