Well, that puts a small wrench into the works in that case, as my tank water is around 7.6 pH, this is basically about the limit for the celestial pearl danios (Celestichthys margaritatus) and also for the Harlequins (Trigonostigma espei) I think the red cherries should be ok up to 8.0pH, going to have to research to confirm that to be sure.
I use fertilized RO water in my tank. PH is typically about 7.4. The limited solubility of calcium and magnesium carbonate limit the effect it will have on PH and GH. In my experience with carbonates is that there is no real risk.
Also calcium and magnesium are weak bases and as a result won't have anything like the effect of sodium and potassium. In fact I have seen PH go as high as 8 when the tank has excess potassium from the use of nitrate and phosphate fertilizers. Both of which contain potassium. I have had blue dream shrimp (they are closely related to cherry shrimp) in my aquarium and they were fine with higher PH.
note the sulfate ion is the key ingredient of sulfuric acid (a strong acid). So if your plants and shrimp use all the calcium and leave sulfate you could have a significant ph drop. In comparison Carbonate ion is CO2 and when that is mixed with water becomes weakly acidic. If you get an excess of CO2 the plants will use it and the rest will evaporate. Sulfur in comparison will not evaporate and will only be removed through a water change.
Keep in mind anything you add to an aquarium will affect PH. Although I have not used calcium sulfate, its high solubility and strong acidic sulfate ion has the potential for significant PH affect. Carbonates in comparison have a weaker base and acidic ions and are self limiting in the water. So Carbonates will have a limited effect on PH.
When I first tried shrimp they kept dying slowly. I concluded it was probably because may RO water did not have enough calcium or magnesium. So I went online and purchased some calcium carbonate powder and magnesium carbonate powder. It is often sold as a food supplement and a 1 lb bag will not cost much. In fact I will probably never use up what the amount I have. At the time I didn't realize how insoluble it was. My shrimp are doing fine now.
I have since concluded that using snail shells, crushed coral, or argonite. is a better way and you might be able to find a years supply while waking on the beach. Just be sure to remove the salt before adding it to the aquarium. I wouldn't use just calcium carbonate or just magnesium carbonate. Typically plants and animals need both and shells will have both and the ratio of calcium to magnesium will stay constant in the aquarium.
Solubility comparison (data from www.wikipedia.org at room temperature)
Magnesium sulfate MGSO4: 350 grams per liter
Magneium carbonate MGCO3: 0.11grams per liter
Calcium Sulfate CaSO4: 2.1 grams per liter
Calcium barbonate CaCO3: 0.013 grams per liter