It may not be bicarbonate, but the carbonate salt that may be added.Pl check it up. Now Na2CO3 + H2O + CO2  = 2NaHCO3. So one mole of Sodium carbonate absorbs one mole of Carbon dioxide to give 2 moles of Sodium bicarbonate. The CO2 produced by the fish is so absorbed by the carbonate salt. This is how the gas level is brought down.

The reason people add sodium bicarbonate (baking soda) is to increase alkalinity. If alkalinity is low, pH is most likely a little low as well depending on other stats of the water quality. Alkalinity is important mostly for inverts and animals that produce chitin. There is an online calculator for dosing your water. It calculates what your pH will turn out to be after adding a certain amount in salt water. 

Hope this helps!

http://reef.diesyst.com/chemcalc/chemcalc.html

Mr. Trivedi, thank you for your answer, it is indeed sodium bicarbonate (baking soda).

Miss Thammahong, thank you very much for you help! :))

Dear Kelvin

It seems that sodium.bicarbonate play different roles in fish transportation water other than pH modification. Please refer to the attached pdf files which could contain useful information.

Best wishes

Dear Kelvin,

I've never heard of using sodium bicarbonate for fish transportation. As you said Kelvin, the higher the pH, the higher the dissolved NH3. So I have much doubts about this.

If you work with freshwater fish and want to add something to the water diminish the adverse effects of stress, first thing is oxygen supply (and adequate transportation tanks of course). Second could be NaCl. This is sometimes used in aquaculture, but not always. Slightly increased salinity contributes to diminish the loss of electrolytes from the fish body in a stressful situation. Generally, not more than 1 or 2 grams of NaCl per liter so as to not get adverse effects from salt itself. But, as always, all of this is species-dépendent. I cannot guarantee anything for your species.