Hi Galen,
I wasn't going to respond to your post cuz it seems that your mom will just empty out the pond every fall into the local river
. However, just in case the fish might be kept, I will add my 2 cents. You said your mom had a whiskey half barrel. I would install a flexible or preform drop-in liner. The inside of most whiskey barrels are charred to add flavoring to the whiskey. This is ash, and will increase the acidityin your water. If you decide to get a liner, be sure to sand the inside really good to remove any splinters. Hyacinths are good plants to keep. You can even put in water iris or even bog plants like creeping jenny (although you may need to put in a cinder block to keep the bog plants from being submerged too deep in the water). Anacaris and Parrot feather are good water plants for the fish. If fish aren't going to be kept, you can go without the liner, but you will need to fill the barrel with water until the wood soaks, and swells against eachother to form a water seal.
Mr. U