Most manufactures require that their products NOT be combined with any other products. One reason is they may contain chemicals that can react very negatively with other chemicals. TankTechsRx has no properties in it that can cause a negative reaction. It can only help, never harm. We do recommend a quick flush of the holding tank for the first tank because we do not know what is in it. The only time we suggest a really through flush out is if a Formaldehyde or Bronopol based product has been used. These chemicals cannot be made nontoxic and can sit in the heavy sludge and kill the bacteria as it is exposed. The easiest wat to do this is to run a garden hose down the toilet and flush it for 5 minutes with the valve open and the hose hooked up at a dump station. Better yet use a TankTool RX to really get it moving faster. Enzyme and bacteria based products will have no effect on TankTechsRx.