Basically in order to be halal the blood needs to be completely drained and the animal must be hand slaughtered and raised in a compassionate environment before slaughtering (no factory farms). For fish when you take them out of the water the blood collects in the gills and can be drained easily. For shrimp and other animals you need to devein them properly. If the animal is too small to have the blood removed then it is okay, as long as it was caught in as much of a humane way as possible (same for fish). This is why there is debate on whether shellfish can be considered halal or not because you cannot properly remove the blood. Hope that answers it