There's a pretty complete one -- a chain -- can't remember which -- on Union Square. Good one called Fish Town way out in Flushing, on Northern Boulevard near Francis Lewis Parkway. Large selection both fresh and salt. Another in Flushing proper, also on Northern Boulevard at Main Street -- salt water selection better than fresh. A Petland with good catfish collection on Northern Boulevard near Marshall's. Coral Aquarium in Jackson Heights on Roosevelt at 75th is a regular stop for me.