Salt tracer experiments are a convenient method to determine travel time distributions in constructed wetland ponds. Typically, these flows are characterized by low Reynolds numbers at times even within the laminar flow regime. In this environment the injection of salt may cause strong density effects, thereby jeopardizing the usefulness of the recorded breakthrough curves. After a tracer experiment has been completed, an indication of potential density stratification in the field may be noticed in the form of surprisingly small recovery rates of a tracer considered as nearly conservative. To provide a tool that permits the intended experiment to be judged at the planning stage already, criteria have been developed that yield approximate maximum concentrations, not to be exceeded if density effects shall be avoided. Laboratory experiments were carried out and the newly derived relationships applied with success. The criteria may in future be useful, too, in the planning of tracer experiments in slowly flowing rivers and streams.