As high as your ammonia is, a 50% water change isn't enough to save your fish. The ammonia level will still be poisonous.
Here's what I would do. Get a large water container, like a kiddie swimming pool or a stock tank and fill it with dechlorinated water to serve as a temporary pond. Multiple containers would be better, but I don't know how much you can handle.. Then use one of your pumps to remove water until your pond is as close to empty as you can tolerate. While the water is going down, scoop out as many of the littlest fish as you can and put them in the fresh water. They will have the best chance to survive since they don't produce as much ammonia as the big ones do. Once your pond is as close to empty as you are willing to go, add fresh water to your pond. Don't fill the pond all at once, but rather fill it about a quarter full, wait an hour or so for the fish to adjust, then fill to half full, etc. Such a large water change is stressful to your fish, but they are swimming in poison, so you don't have a choice.
I certainly hope you got a test kit at the pet store, because you will need to keep testing the water every day to determine ammonia and nitrite levels and changing water to keep them down. When your tests start to show nitrite, your ammonia level will start to go down. That's good, because it means the bacteria that change ammonia to nitrite are established in your pond. Unfortunately, nitrite is also toxic, so you will have to continue the water changes until it disappears. When the nitrite goes away, you get to relax a bit.
Would you please tell us what kind of filter you have -- brand and model -- so we can be sure that it is adequate for your pond?