An alternative "detergent and water softner" for the GEO method is "Trend" brand detergent from Dollar General. I use a scoop everytime we start our black tank fresh, along with ~5 gallons of water. It works very well, and my sensors have always worked once I started with this method. The other key is good oxygenation. Rarely do people flush their vent stacks. I have found that in the south if you don't have screens on the vents, the mud daubers will build nests in the vent stacks. This creates a "anaerobic"/low oxygen environment in the tank and can help lead to the "slime"/sludge buildup that fouls the sensors as well. Once I flushed the vents stacks (literally with the wand sprayed on the end of the water hose), I screened over them before I replaced the vent covers, and all has been well. Since I started the GEO and the vent clearing at the same time, I am not sure which had a better outcome.
Current TV:'14 F350 Lariat CrewCab, 4x4 Short Bed SRW Diesel w/ Pullrite 2700 (16K)
Current Fiver: '15 Puma 295BHSS Anniversary Edition
Past TV: '13 F150 HD Payload SCrew EB, '08 Tundra CrewMax 5.7L, '02 'Burb 5.3L, '97 Dakota 3.9L
Past Trailers: '09 Wildwood LE 27RB, '02 Palomino Yearling RL, '87 Jayco 1006dlx