I get little neck clams from my local sea food place and freeze them. A 5 minute soak in tank water and pop em open with my thumb and toss it in. All my fish go crazy for clams.
Fresh seafood generally just means whatever you can find that's fresh. If you can find halibut and fresh rock shrimp, then yeah. It's also fine to use frozen seafood for human consumption (although you might want to rinse it first). My fish love a mix of bivalve + squid + shrimp, which is typically bought "fresh" (it's on ice), then frozen, then minced/blended.