A system for a fluid tank for storing a urea solution in a motor vehicle, the system including a first float having a first density and a second float having a second density, the first and second floats being movably arranged on a guide in the fluid tank, the first density is less than a density of the urea solution at a reference temperature but greater than a density of water at the reference temperature, and the second density is less than the density of water at the reference temperature. The system further includes a control system configured to receive signals indicative of a level of the floats, and to generate an anomaly signal in response to detecting sinking of the first float and floatation of the second float.