Tannin is a chemical given off by decaying matter that falls into your pond. Tannins and other debris can leach the oxygen out of the water as they decompose, and also turn your water an ugly brown color. There are a number of tricks you can try to remove tannin from a pond. If you find that one of these methods isn't enough, try another one or even a combination.
Fill up the child's pool or bucket with water from the pond. Catch the fish in the pond with a net and transfer them to the bucket or pool. Using the old water will help prevent the fish from being shocked by new pond water.
Remove any rocks or large objects that may be on the bottom of the pond. Use a plastic rake to scrape up any pond debris.
Turn off the power to any filters or pumps running in the pond.
Install a UV filter in line with the water flow. These are small light bulbs placed inside a length of pipe. As the water passes in front of the bulb, the UV light flashes it, killing bacteria, algae and other chemicals, such as tannin in the water. These are usually placed in between the pond skimmer and the filter, so you will need to cut open the skimmer pipe with the hacksaw in order to obtain the best UV position. Glue new PVC couplings in place and secure with the primer. These UV filters also require a power outlet in order to work properly.
Install an active charcoal filter in conjunction with, or instead of a UV filter. There are a number of different types of these filters, but they basically all run in the same fashion: water runs through a central chamber where it is filtered through a bag containing the charcoal material. No electricity is needed for this, but you will have to cut into the water filter line with the appropriate tool.
Restore the power for the pumps and other features. Check the pipes on the new UV and charcoal filters to make sure the pipes aren't leaking.
Replace the fish to the pond.