Goslings dying HELP!

[Abundantly Blessed]

I have a flock of teen goslings (7 weeks). I've lost 6 in the last week. All were found dead in the morning. No signs of illness like runny nose etc.. they seem to start out with weakness, preferring to lay down but will walk and eat. Then they stop eating and drinking.... then they seem to have seizures or seizure like spasms at first spread out, then several in a row until they die.
They are in a chicken tractor and are moved everyday. Fresh water twice a day with niacin supplement. They are fed a homemade chick starter mix at 21% protein to supplement grass as they finish feathering out. The feed contains: wheat, barley, field peas, flax seeds, fish meal, and Fertell poultry mineral mix.
I am questioning the fish meal.... some people claim it could cause mercury poisoning.... but commercial feeds use it for poultry. Is this a mistake? They have been fed this feed since hatch. Feed is 10% fish meal.
P.s.... I did a 1 necropsy on one of the more recent deaths and the liver looked healthy and of normal color. I did not notice any other abnormalities in any of the other organs.
Thanks for any help or input!

 

[Pyxis]

Are they living outside? Is there something that they could be getting into that could cause this? Like weed-killer treated grass or something along those lines?

Perhaps try switching to a store-bought feed such as Naturewise chick starter for a bit to see if the deaths stop. If they do, then we know it was something in the feed.