Here's the curve for aluminum solubility as a function of pH in fresh water.
It is for aluminum hydroxide, so the data for aluminum oxide will be the same curve, but moved straight down becasue it is a more stable crystal form. That said, I don't think we know what crystal forms Phorguard shows on its hydrated surface, so it would likely be between amorphous corundum and amorphous aluminum hydroxide.
I believe it is correct to cross over the data at pH 4 (where we have data for both forms), as I mentioned above, giving solubility for corundum around 0.1 ppm in fresh water at pH 8.2
