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I apologize that no one has responded to your question sooner. Different experts come online at various times. I just came online and saw your question. My name is ***** ***** I’m a biologist with a special interest in avian health. I'm sorry to hear of Daffy's problem.
I feel bad to give you less than good news, but I believe you deserve honesty. The vet you saw did all that is possible. The combination of symptoms you are seeing - seizures, little appetite, failure to grow properly, and falling over when swimming - point to a serious neurological disorder. It was probably genetic. Such things happen in a percentage of young animals. With ducks, it's not unusual to lose about 10% of a clutch. That doesn't make it any easier to see when one of our own just doesn't thrive. But it does mean that nothing you did or didn't do caused the problem.
There is probably nothing else the vet can do, but I suggest you make a return visit. Discuss with the vet the possibility I just explained. It is likely that Daffy will continue to decline. The best thing to do might be to have the vet humanely end his suffering. Sometimes what is so difficult for us is most humane for our animal companions.
If you have more questions, just let me know. I'm sorry that you are going through this with Daffy.