Play sand is usually too light to be used as tank substrate, it takes a very long time to settle, and most people don't have enough patience for how long it takes, it is also easily stirred up and can ruin filters. Pool filter sand is heavier and stays put, so the water doesn't cloud up.
You can use the vinegar test on rocks too, pour it on the rock, if it fizzes it means it is made of calcium carbonate and will dissolve in your tank making water cloudy as it comes in contact with acids from fish waste and urine.