Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
It sure shouldn't. I had a 125 gallon salt water tank that I used for fresh water after thorough cleaning and there was no issue at all.
Look into the play sand. It might be a limestone based sand, and the minerals in the sand could be contributing to the GH, KH and pH.
Test a handful of the sand in a separate jar of tap water. Test GH, KH, pH and TDS when you start and every few days for a week or two.
Do the same test on rocks or other things in the tank.
pH is not a stand-alone value. It is affected by other things in the water.
If the KH is fairly high then the pH tends to be high, and stable.
If the KH is low then the pH can vary quite a bit, depending on what else is in the tank.
Generally organic matter and organic processes such as decomposing fish waste, fallen food, old leaves and so on create an acidic reaction. Adding CO2 usually makes an aquarium more acidic, but in a daily cycle as the plants use varies night and day.
Carbonates and bicarbonates in certain fertilizers will add to the KH and this generally makes the pH rise.
The link between KH and pH is not a hard and fast rule, but a pretty decent guide as the first thing I would check to see what is going on with the pH.
If you can post GH, KH and TDS levels of tap water and tank water that might help us to figure out what is going on.