The best readily available fish white meats (IMO) are halibut, cod, and pollock. these are known to be virtually free of mercury toxicity, are high in omega 3 (moderate omega 6 levels), low in fat, and for aquaria, are low in oil content that can foul tanks and filtration parts.
Oddly enough, these are the more popular species used in fried fish fast foods because of the meat's flakiness and low 'fishy' taste.