If you would like to learn from an expert where to buy fish/seafood (and much more) on a local market:
Gisela, Head Chef of Tepui Restaurant, gives a tour of Central Market showing where she buys fish for her restaurant, as well as how you can recognize whether it's fresh etc.

Wednesday, 30 April at 11am, price: $20 including a local lunch with delicacies like grilled seafood, (curry) noodle soup, spring rolls, vegetables and fruit at the food stalls.
Booking: info [at] expat2cambodia [dot] com
More info:http://www.expat2cambodia.com/blog.html