The paper deals with the construction of the output feedback controllers for the systems that are transformable into a simpler form via coordinate change and static state feedback and, at the same time, via (possibly different) coordinate change and output injection. Illustrative examples are provided to stress the major obstacles in applying the above scheme, especially as far as its global aspects are concerned. The corresponding results are then applied to the problem of the real-time control of the water-storing plant. Using the methods developed in the theoretical part of the paper, the control of the water levels is designed to handle the unknown influx of the water into the first tank using measurements of water levels only. Simulations results are presented showing good performance of the designed controller. Some preliminary laboratory experiments have shown promising results of the real time implementation as well.