Bernard, sometimes it could be a form of ionic imbalance if you can't seem to bring your cal or alk up. Perhaps a lack of magnesium? Or perhaps in PH? Maybe you can do up to 20 to 30% water change to see if it helps?
Try to dose buffers and measure the drop the next day. Are your corals really growing that fast esp if they are SPS type? It's possible for an SPS tank to see huge drops in Cal and Alk due to the uptake by corals.
But first, get good test kits because a faulty set could cause you to overdose/underdose cal and alk supplements and an imbalance can actually cause a drop.... do read up on reefing chemistry... Cal, Alk and PH are all closely tied to one another.
AT