For what it's worth, I found an article on supplementing with omega-3 fatty acids (fish oil) to help in cases in which drugs are not enough to control epileptic seizures:
Scorza et al. 2009. Positive impact of omega-3 fatty acid supplementation in a dog with drug-resistant epilepsy: A case study. Epilepsy and Behavior 15:527-528.
ScienceDirect.com - Epilepsy & Behavior - Positive impact of omega-3 fatty acid supplementation in a dog with drug-resistant epilepsy: A case study
The article describes a Great Dane whose epilepsy was not completely controlled by phenobarbitol. The addition to the diet of only 2 grams of omega-3 fish oil per day (which isn't much for a dog the size of a Great Dane) reduced the frequency of seizures by 85%. I don't think that the article is behind a pay firewall, but just in case it is, I'll quote a few selected paragraphs below (fair usage):
Abstract
Epilepsy is the most common neurological disorder in both dogs and humans. Although the pharmacological options for treatment of epilepsies have increased, it has been reported that two-thirds of dogs with epilepsy are refractory to antiepileptic drug therapy. To our knowledge, there are no experimental studies in the literature that show an effect of omega-3 supplementation on epilepsy in dogs. Our case study describes the effectiveness of daily intake of a moderate amount of fish oil in a case of canine epilepsy.
Introduction
[..]
Epilepsy is [..] the most common chronic neurological disorders in dogs, with a prevalence of 0.5 to 5.7% [3]. In most dogs, recurrent seizures have no identifiable underlying cause, and these cases are classified as idiopathic [3]. It is important to note that two-thirds of canine epilepsy patients are refractory to therapy, despite the availability of new antiepileptic drugs [3], [4] and [5].
[..]
There is currently no evidence that omega-3 fatty acids, either alone or as add-on therapy, are therapeutic in canine epilepsy. This case report describes a substantial reduction in the frequency of bouts of idiopathic epilepsy affecting a female Great Dane when omega-3 fatty acids were added to marginally effective phenobarbital background therapy.
Case history
A 2-year-old female Great Dane with a history of recurrent seizures was brought to a veterinary neurologist. Tonic–clonic seizures occurred at a frequency of three times per month [..]. After a detailed physical and neurological examination, evaluation of hematology and serum biochemistry, and magnetic resonance imaging of the brain, the veterinarian diagnosed the condition as idiopathic epilepsy and began treatment aimed at seizure control. A trial therapy with phenobarbital (2.5 mg/kg, twice a day orally) failed to adequately control the seizures: after 8 weeks of treatment there remained two seizure episodes per month. Typically, potassium bromide is used to supplement phenobarbital in refractory cases. However, in this case, it was decided to supplement the dog’s diet with moderate amounts of fish oil (oral omega-3 polyunsaturated fatty acids, 2 g/day). The frequency of the epileptic seizures markedly fell after 50 days of combination therapy with phenobarbital and omega-3 fatty acid. During the subsequent 18-month period, seizure frequency fell to one per 3 months, a reduction of about 85%.
Discussion
[..]
From an experimental point of view, we demonstrated previously that chronic treatment with omega-3 promotes neuroprotection and increases the number of parvalbumin-positive neurons in the hippocampus of rats with epilepsy, suggesting that omega-3 promotes plastic changes in the brain [9]. The case study reported here is compatible with this idea, and suggests that omega-3 intake may be an option for the treatment of epilepsy in dogs.
[..] It seems that the development of new therapeutic strategies like the addition of omega-3 fatty acids to more conventional treatments should be explored in the treatment of dogs with epilepsy. However, it is important to note that nutritional therapy (including omega-3 supplementation) is not a substitute for conventional anticonvulsant medications.