There are 2 main water salts added to RO water to build up a water profile. Calcium chloride and calcium sulfate. Both will add calcium to the wort which is needed to help the yeast, specifically to flocculate and it reacts to the phosphorous in the grain to create ions that drop the pH. You should target at least 60-70 ppm or more of calcium. After that, it depends on the beer.
I brew both malty and bitter beers. Malty beers benefit from chloride, so I target @100-150 ppm of chloride. Then I add some sulfate to balance them out. I end up with 2-3 parts chloride to 1 part sulfate. In a bitter beer you want to do the opposite, target 100-150 or more of sulfates and then a addition of chloride to get a 2-3 parts sulfate to chloride ratio. In a balance beer (honestly most beers fall into this category) target 100-150ppm or so of both. These are only rough guide lines, increasing the sulfates will benefit hoppy/bitter beer, but the level varies from brewer to brewer depending on their opinion and tastes. It's safe to say that an excessive amount of either will tip the beer one way or the other, but the top for chloride should be @ 200ppm and the top for sulfates should be @250-300ppm. There is such a thing as too much water salts in beer.
The other salts that are added are magnesium, calcium hydroxide (pickling lime), calcium carbonate (chalk) and sodium bicarbonate (baking soda).
Magnesium works similar to calcium in dropping mash pH. I generally don't use it because there is a lot in malted barley and it's not as effective as calcium.
Calcium hydroxide and sodium bicarbonate both are used to add hardness to water, to basically bring up the pH. If you brew long enough, you realize it's easier to drop pH than to raise it. In my opinion, I would only use these when absolutely necessary. Baking soda is the most effective means of bringing up pH, but it brings sodium to the beer. It's okay in some circumstances, but too much and the beer can go south fast. Use baking soda with caution. I haven't used pickling lime, so I can't say how well it works, but I know it's not as effective as baking soda.
The last and least is calcium carbonate (chalk). It does not dissolve into water without the presence cO2. So most, if not all the chalk that's added to water precipitates out of solution. It doesn't raise pH much, if at all.
Switching to RO is a good idea, but it's not a magic bullet. Darker beers need hardness to keep the pH up, so I blend my hard tap water into the RO to bring up the hardness. Depending on the beer, I may blend 50/50 or more. In order to do this you will need a hard water with a known hardness.
Dropping pH is the easiest and can be done with lactic or phosphoric acid in the mash, boil and even in the finished beer. Sparge water can also be dosed with acid to prevent tannin extraction from the mash and to keep the pH in check during the second runnings. If you are going to wade into the pool of water modifications, it would be best to have a reliable pH meter to help guide you along the way. It can get pretty complicated at first, but after a while it will get to be second nature. It's one of the last things most brewers do to improve their beers, but it can make a significant difference in the final outcome of the beer.
