One of the problems with soft water systems is that the hardness tends to keep the pH at a certain level. Without it the pH tends to swing more easily with natural acids and such like.
This is great news for the hard water keeper (although they have their own water issues) but doesn't mean the making the water harder is necessarily the solution, especially if you accidentally overcook the correction and end up with hard water.
Also, water changes become an issue with tampered with systems. You add coral, the pH rises, you water change, it drops again, then climbs during the week.
If you want to solve a pH crashing system then you need to look at the water going in, or decaying organics that are crashing the pH, and my old mantra, that anything you do to the water you need to be willing to keep doing.