Hi, I also have the same gauge on my holding tank and for 3 years thought it was duff and didn't bother with it - until the tank overflowed into the boat! Having cleared up all the crap and taken the thing apart I discovered that it had been working all the time, and heres the good bit - it only works at the top 25% of the tank! So, you fill the tank to 75% and the guage still shows naff all, but then the last few pumps will quickly push it up to full. To check that you have the same problem measure the lift of the float from fully down to horizontal and check that against the depth of your tank, or try filling the tank with water down the pumpout and sight the water in the tank before refitting the gauge for a reading.