The reason cement raises pH is because it leeches calcium into the water. In small doses, this doesn't harm fish. It's actually really good for some fish. African cichlids thrive in it. Here in PA some of our best bass fishing is in old strip mine ponds, which are extremely basic and hard from calcium leeching out of the limestone walls. It's not man-made chemicals leeching into your water, or necessarily even bad.
Do you put driftwood in your aquariums? Driftwood makes water softer because of those tannins.