Most things we use as preservatives are poisons- the whole point is that they kill/prevent the bacteria that cause decomposition.
It's just a matter of how much, how it's metabolized, and if it will build up rather than pass through a fish (or human) system.
Some people do make the choice to go all organic and eliminate all but organic preservatives all together. I personally think there is a lot to that... I eat mostly organic food, myself, and I'll buy organic options for my fish/pets when there's an economical/practical choice available.
It's just a matter of how much, how it's metabolized, and if it will build up rather than pass through a fish (or human) system.
Some people do make the choice to go all organic and eliminate all but organic preservatives all together. I personally think there is a lot to that... I eat mostly organic food, myself, and I'll buy organic options for my fish/pets when there's an economical/practical choice available.