Fish that have thaiminase are deadly to tropical predators because it destroys thaimin (?) which I think is a B vitamin. So right before you feed feeders goldfish why can't you gut load them which thaimin injected food so it will cancel out?
Read the above and you can draw you're own conclusion. IMO if you feed a quality vitamin enriched pellet as the staple and don't over do it, it should not pose a problem. If it is the sole or main constituant then it could become a problem over time based on the frequency and quantity ingested.