BMAA is the neurotoxin (implicated in Alzheimers and Parkinsons). It also bioaccumulates over time, and potentially up trophic levels. So definitely not fish wonder food until there's some serious testing done.
Edit: researchers are still studying the issue, but it seems some plants may be capable of concentrating BMAA, which is highly water soluble. Wheat seems to capture it in proteins of the shoots. I haven't found any further specifics other than cycads, which have a cyanobacteria symbiosis and concentrate it in their seed coatings.