Stay away from the ATK, it’s one of the worst designs for an ATO out there, especially compared to the Tunze. Just buy the new updated version of the osmolator and enjoy your tank. I’m still using mine (older version), which is 8 years old now. I just replaced the optical sensor to give it another 8 years + of service and it only cost me $38. The Tunze is the gold standard for a reason.