If you want equipment, there's a few places near the area u mentioned. At Sin Ming area, there's Reef Depot and Madpetz, in Serangoon North, there's petmart and in Yishun, there's a aquarium shop near central. Of course, there's also numberous aquarium shops that sell freshwater fishes and equipment.
For livestocks, you can go GO at Upper Serangoon Road for corals, AM is at Upper Paya Lebar and there's Sealife at Balestiar area so there's still some choices. However, if you really want fishes/corals, farms in Pasir Ris, CCK and LCK are still the best choice.