Sea creatures on another planet are not suitable for human nutrition - looking for a simple explanation why not

There is a group of scientists doing research on another planet which may well be human habitable. Most of the life is concentrated in the oceans. The variety of fish-analogues and other aquatic creatures is huge. Unfortunately, they cannot be used for human food.

I need a simple, scientifically solid explanation why not (the real reason is that storywise it should not be too easy to settle on another planet ;) To make it more complicated, there is a family of creatures that are biologically distant enough from the rest to make them edible by humans. Thus chirality of amino acids would not explain why it would be frustrating to go fishing.

EDIT: thank you all for so many suggestions! It has been truly inspiring to read them. I hope that if someone else has been wondering about similar things they have gained new insight, too.

What amazes me is how lazy people are: dozens of people never bothered to finish my original post which was seven rows long. In the end I say that the chirality of amino acids would NOT be an explanation here. I lost the count when I was trying to see how many suggested just that. They had just read the first few lines and rushed to write their suggestion like an attention-seeking kid in school "Me! Me! Me! I have the answer!" :) :) :)