5 degrees of KH is plenty for using CO2. The problem with using the crushed coral is that now the KH will go up slowly as long as the coral is in the water. So, you will always have to remeasure KH as well as PH to determine the CO2 in the water. I haven't found that adding baking soda only gives a temporary boost in KH. I add a half tsp at each water change, don't bother measuring KH for a day or so, and it stays constant then.