Common pests found in nursery ponds include unwanted fish, predatory insects, or stubborn algae blooms. Extra fish can be introduced to your nursery pond by eggs delivered through outside water, birds, insects, or other various forms of transportation. These can take up valuable resources needed for your actual fish, or can even eat the fry if allowed to become big enough. This can be solved through manual removal, dewatering the pond, allowing for refreshing periods between stocking, or the use of poisons.
Some insects directly prey on fish fry, while others are simply an unneeded drain on limited resources. A thin film of soil or soap emulsion over the surface of the pond will suffocate insects; just be sure it's not so heavy to harm fry or the plankton they're trying to eat!
