Songkhla's aquarium is located near the very tip of the peninsula (see map), a bit removed from the other sights of the city although it is right next to the park at the very tip of the peninsula. The collection and displays are rather good, but it's not the most politically correct place. There's a go-cart track in the parking lot, and the fish-feeding show in the big tank is downright creepy.
The Aquarium is open daily from 9:00 am to 4:15 pm. On Friday and Saturday it's open until 5:15 pm and on public holidays it stays open until 6:15. Admission is 200 Baht (5.19 USD) for adults. For additional information, see the Songkhla Aquarium web site.