Cement tank is a tank made of concrete. It is primarily used to store water. We do convert it to store our pets. Infact, all the ornamental fish farms in our region use these tanks to keep fishes. The fishes grow very fast when kept in these setups. I don't know why!
As Lokky said ornamental fish farms use big cement tanks usually indoors to raise and breed fish. Usually along with the well known glass tanks. A long time ago they used to use cement-asbestos tanks for fishkeeping instead of glass (its in my dads fish book from the 70's) by that time they were out as they were heavy and eventually wore down etc and glass was in.