Just a bit of info I gleaned from an old fish-expert down at the LFS.
When using crushed coral to raise pH, follow these simply rules:
1) Use 1/2 cup of crushed coral per 20 gallons
2) Wash it THOROUGHLY before using it
3) Put it in a bag/stocking and place it in your filter. At first it needs to be 'filtered through' flowing water to work well. Once your pH has balanced, you can scatter it through your substrate if desired or leave it in the bag.
4) Crushed coral only raises the pH until it hits 'equilibrium' - being between 7.4 & 7.8. Once it hits that, it is rock solid and will not change.
5) It takes 2 - 3 days for the coral to work, leave it a week to be safe.
6) Once you use the coral, you never have to redo it - it will stay balanced. If your pH starts to drop, the coral automatically adjusts it by releasing more calcium. Your pH will NOT rise above the equilibrium either.
7) Don't let your LFS kid into thinking crushed coral is 'special and expensive' - it's just marine tank substrate... nothing more.
8) You can also use a shell from the beach/seaside. Be careful of ornamental ones, they may have been bleached - they will leach bleach and KILL your babies
9) Boil any shells you find on the beach for 30 minutes. A couple of palm-sized ones or a handful of smaller ones equals 1/2 cup of crushed coral.
10) When doing water changes of 50% or over, leave 2 tablespoons of coral in your bucket in a bag (can stay there always) for leaving your water out over night.
Amazing what info you can get when you smile sweetly
Oh... if anyone in Australia needs a couple of cups of crushed coral... I ended up with a 5kg bag (doh!) I will gladly mail some out for a couple of dollars just to cover postage.