[Snaky]

@abcat, nessy:
Thiaminase is an enzyme that will tackle vitamine B1. This does not mean that in nature gartes won't eat those fish, they just eat enough diversity to get the vitamine B1. So it's not a problem to give them fish that contains thiaminase if you supplement it with vitamine B1, which I get for free (powder form) at the local pharmacy.

edit:
You can also kill the thiaminase by heating the fish to ( I think ) 80 °C.

And the list of fish that contain thiaminase and those who don't is handy.