Sorry to hear about your duck.
Seizures are a little tricky topic with birds, there are a few wide varieties of conditions and diseases that can cause seizures, ranging from neurological damage, nutritional deficiencies, to be more specific vitamin E and thiamine (B1), toxin ingestion, metabolic diease's, and possibly neoplasms. At her age, toxin ingestion, neurological damage, or a nutritional deficiency would be more likely.
A vet will likely be able to perform several diagnostic tests, to further narrow down what might be the underlying cause of the seizures with your bird, so I want to put emphasis that if you can see one, you should. Your duck has the best prognosis in the hands of one.
I know money is an issue with a lot of people, avian vets are very common either so we can try our best to help to the best of our ability.
A thorough anamnesis would be a great place to start.
What is she being fed, is she crested, are you aware of any damage that was done to her head or neck area, have you had any problems with your birds in the past, please further describe her seizures, Is there any consistency to them, how often does she have them, Do you know of anything she may have eaten, does she act normal otherwise, do you feed her any treats, could you post a video and picture of her and your setup that you're keeping her in?
Even if the underlying cause isn't nutritionally related, sometimes the affected bird's seizures will subside with nutritional support and supportive care. If she were mine, I would start her on Cattle B complex, and give her 1ml daily, either over treats or orally. In addition to that, I would start her on vitamin E 400iu capsules, you can give her one of these a day, either over treats or orally.
Regarding administering oral fluids, and dosage's read these links,
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/
https://www.backyardchickens.com/articles/thiamine-deficiency-in-waterfowl-in-progress.75690/
The level in which I would provide basic supportive care would depend on the severity of her condition, if you further describe her seizures more, maybe I can go into what more advanced supportive care may be, but just for basic care, offer her feed and water often, do supervised water therapy, keep her on soft bedding, and avoid any deep water dishes in her play area that she could possibly drown or aspirate in.
CBD gummies, may help?, but they are not addressing the underlying condition, so IMO shouldn't be used.