Besides the crushed coral, I would add baking soda to buffer. If there are fish already in the tank, start by buffering slowly over the coarse of a several days. You'll want to add approximately 30 tsp (1 per 10 gal) or 10 TBS of baking soda. Mix 3-4 TBS in a couple of glasses or bucket with dechlorinated water and add to the tank. Do this slowly over the span of several days to get the pH up to the desired level (around 8.2)
Add the equivalent amount to replacement water when doing water changes.
This is, of coarse, a rule of thumb and works for my water. Your water is much more acidic than mine and may require a bit of testing to fine-tune.