A better approach would be to add crushed coral to the substrate or get a large piece of decorative coral, which might do the trick for a tank that size. I did this completely by accident in my early days, before I understood that shell material is not inert. It is easy to screw up when using chemical means added to the water to alter your pH in a small tank (or a big one, for that matter!). Every water change has to maintain the altered pH when you do it that way, if your tap water is different from your goal. The change in pH is much more stressful than keeping them in pH that is slightly lower than their native waters. Are you planning on keeping Africans?