Soda ash is more convenient than bicarb as you don't have to bake it and it will buffer PH also.

Dosage: Add 1 teaspoon (5 g) per 170L to raise alkalinity by approximately 1 dKH. Dissolve appropriate amount in RO/DI water, then slowly add solution to a high-flow area, ideally into a sump. Monitor pH while dosing. Avoid immediate impact of pH change to more than +/- 0.2. Re-test and repeat dose until desired level is reached.

For more details on DIY dosing we recommend using Randy's recipie which can be found here...

http://reefkeeping.com/issues/2006-02/rhf/index.php