Originally posted by Faramir
I strongly suspect that it is your infrequent water changes that are at the root of the problem. Your nitrification process uses up the carbonate buffer, allowing the pH to drop. This can be replenished using coral sand, but you would do better to increase water changes in order to dilute nitrate and dissolved organic compounds (DOCs).
Agreed, meager water changes every three weeks is definitely not enough for a 29 gallon with that kind of fish stock.
Do 30-50% every week and then if you still have pH troubles, start doing the crushed coral thing.