Epsom salt is just magnesium sulfate. It's used for soaking injuries like sprains, as well as a laxative. It's also used a lot for horses and other animals. So if you can't find it at a local pharmacy or grocery store, try calling a veterinary clinic. It should NOT be expensive. Look for 100% magnesium sulfate. (No dyes, perfumes or additives.) To give you an idea, it costs about $1.50 per pound in the US. (About $1.50 USD per 0.45 kg.)
(Aquarium salt is generally used for external issues. Epsom salt is used for internal problems, such as bloating.)
Edited to add: Do a Google search for "Epsom salt Hong Kong." When I did this, the results came up with several forum discussions on where to find epsom salt in Hong Kong, and also listed several pharmacies that carried it (but I don't know how far they are from your location).
Last edited by LittleBlueFishlets; 02-24-2013 at 01:27 PM.