Adjusting water after fermentation

I recently brewed an oatmeal stout with fairly soft water.
Chloride 10 ppm
Calcium 13 ppm
Magnesium 4.9 ppm
Sodium 13 ppm
Sulfate 16.4
Alkalinity CaCO3 50
Hardness CaCO3 54
pH 8.8

Mash efficiency seemed to be fine, what I'm worried about is the harsh taste I've heard associated with brewing darker beers with soft water. Can I adjust after the fact?

I think the theory is that dark beer brewed with water that is not alkaline enough are acrid. I don't believe that and recently scored low-mid 40s with an American Stout that was brewed with water less alkaline than that.

But I guess the question is what do you believe is wrong? Beer is too acidic? Not enough of some ion? You can fix those easily with salts or potassium hydroxide. You would have to severely water down your beer to strongly effect either parameter by blending water into it.

The stout turned out great, the pH of the mash was spot on, I'm guessing that the carbonate issue is more of a problem for people with closer to normal water.