I live in Long Beach (but used to live in Suffolk Co.)
PETS used to be good, once they expanded they went down the tubes.
Aquarium Adventure is a new place in Carle Place on Glen Cove Road - it's absolutely amazing in there. They could charge admission. LOTS of variety, fish seem to be in really good condition, but alot of the staff isn't very knowledable. Their plant tanks are set up really nice.
Pet Barn is located in Franklin Square on Franklin Avenue. It's an old school mom & pop store that's pretty big. Really knowledgable staff (especially the guy w/ the goatee and glasses). great place to pick up equipment. they have all sorts of old school equipment tucked away that they'll sell you on the cheap. good selection of fish and plants, too.
if you're ever out in Suffolk:
Magic Isle in Centereach - good selection of stuff. some of the employees are really knowledgable.
my all time favorite: Country Critters in Patchgue. They're huge, have an amazing selection and are really knowledgable.
there ya go.