I had the same problem with my level indicators until I had the tanks professionally cleaned by a company in Phoenix that specializes in holding tank cleaning. The technician that cleaned the tanks told me that the grey tanks almost always have the most build up in them because people tend to leave them open most of the time and the soap bubbles carry grease and other debris down into the tank and deposit on the sides. The device they use to clean the tanks allows you to see the sludge that comes from the tank and it is amazing how much stuff gets flushed out. The company is called Tank Techs 623 566-1533.