@Kaizoku_Ou:
True, but that was the fish's own poison, not some toxic waste that it ate and survived.
My point is, if a big boar and tiger can survive that "toxic waste" then Zoro will more than likely survive it. From what we saw of him this chapter he already seemed used to it and it doesn't bother him.
I think one bite from that fish would kill Zoro instantly.
Also Tama just fainted and after was able to speak to Luffy so the poison in the water isn't THAT dangerous. And what happend in Impel Down to Luffy has given him some of the best antibodies possible against toxins, regardless of origin. The composition of toxins differs from one to another, but much of the elements are common and that is why Luffy can resist almost any type of toxin. Now about the poisonous fish … there is venom and there is poison. If you bite it and you die, it's poison and if it bites you and you die it's venom. They are much alike but the difference is where the toxin gets in the body. The venom injects the toxin from outside into the body through wounds, skin, etc but the fish injected the poison because Luffy directly digested it and it went into the stomach and to the vital organs - remember how he looked like when Magellan poured his Venom-Venom DF all over him and he was dripping venom from his mouth ... Level 5.5 had to exhaust all their food supplies to keep him alive.
So there is the difference: Luffy can resist almost any toxins from outside the body - gas, stings, bites, etc - but if he ingurgitates something poisonous, his body has a much tougher battle against it.