DO NOT MISS THE CHANCE TO SEE THE HK FISH MARKET!!!!
It's incredible, and will change your perception of what an aquarium store can be. Entire stores are individually devoted to reefs, planted tanks, Discus, Goldfish, etc.... Each one more amazing than the last. It is also a complexly different place than the commercial fish market pictured above.
The last time I was there was in the infancy of digital photography, so I unfortunately don't have any pics to share.
The bagged fish are there, but they are bagged daily in pure oxygen and are snapped up quickly. Due to the proximity to all of the area fish farms, and how seriously the hobby is taken there, the bagged fish were among the fattest, brightest and healthiest pet store fish I have ever seen. You will not be disappointed by what you find there. I was almost embarrassed when discussing what US shops are like with some of the vendors. They all speak perfect English and are very nice. The one thing you need to remember is to not take pictures without asking first if it's ok...
The Kowloon peninsula and island area also have a ton of other interesting things to do along with incredible food and nightlife. I would highly recommend the 2 day detour.