Beginning on July 1, 1979, in addition to the taxes imposed by the Illinois Income Tax Act, there is hereby imposed upon the water companies subject to the taxes imposed by the Illinois Income Tax Act, a tax in an amount equal to .8% of such water companies’ invested capital for the taxable period. If any such water company is not liable for the invested capital tax for the entire taxable period, such invested capital tax shall be computed on the portion of the taxable period during which such water company is liable for such invested capital tax. The invested capital tax imposed by this Section shall not be imposed upon persons who are not regulated by the Illinois Commerce Commission and who are subject to the tax imposed by this Act only with respect to transactions between the seller and tenants of buildings owned or operated by the seller.