You have to have at minimum 10 of each fish that you intend to stock the pond with before it will allow you to stock a pond with that particular fish. Check your inventory and see how many of each fish that you have.
It doesn't matter if one of your family has enough. The number of fish is not cumulative in terms of family total. Each individual person has to have 10 fish to stock the pond. The other alternative is to transfer the number of fish that you need to stock the pond from the others who have stock of that fish in their individual inventories to the one who you want to stock the pond.
Stocking a pond comes from the individual inventories. It will not check the family inventory.