Nursery ponds can come as excavated or compounded reservoirs, or be formed by trapping fish within a mesh pen or cage. An excavated pool can be dug out of the earth, or can be in the form of a preexisting pond. You'll need to use a waterproof liner to prevent water from seeping into the ground, and to protect the reservoir bottom from erosion or cracking. A compounded nursery pond can be as simple as a 10 gallon drum, as long as the water within is clean, and the material is sure not to leach any toxins that may harm your fish.
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.
