The ideal type of nursery pond for your set-up will depend on the kind of fish that you're wanting to help grow, and the size and conditions you're working within. An excavated pond will probably be cheaper than building a container, but will require a watertight liner and will likely be exposed to the elements. Controlling conditions within a natural pond may be somewhat more difficult, but water will naturally evaporate, making water-changes a matter of simply adding fresh water.
If you want to create a nursery pond within a larger system, an isolated nursery reservoir can be created via mesh curtains that allows water to pass through the pores without allowing fish to escape. This makes controlling water conditions harder, due to the expanded pool that the fish live within. However, this eliminates the need to create another isolated pool.
