We had several Cavalier King Charles Spaniels experience seizures of different severities over the course of a summer a few years back. We tested the dogs, but they would only have one event, and it was in multiple dogs. The blood tests came back completely normal. So we started looking at external causes. Disinfected the kennel room. Bleached the kennels several times. Changed foods, even checked for spiders in and around the kennel. The vet came and inspected and found nothing that could be causing the problem.
Noticing that the problem quieted down after the summer passed, we started to do some research. What we found was that in California, garden hoses are labeled with a warning that unless specified your typical garden hose contains toxins. Apparently if they sit in the sun and under pressure this can concentrate the toxins enough to cause seizures in small dogs and puppies.
We immediately switched the water supply for the kennel to a RV type (white with a blue stripe) hose which is designed for drinking water. Have not had a problem since.