I would say your circuit is loaded to the max you need another plug-in installed that does a home run back to the breaker box. when you ATO kicks in it loads the outlet and draws the operating power below what the PH can operate at. I think you have everything on the same circuit and breaker. your plug-ins behind your tank are not split plugs wired to different breakers. If you turn off that breaker how many individual things turn off this includes on your tank each heater is an item and a wall socket that is also considered an item as some thing can be plugged in to it at any time. I believe that you are only supposed to only have 11 or 13 item working on a circuit this you can look up on this you can look up on the net.also install a GFI plugs for tanks any plug that is within wa ter zone has to have GFI plugs. If some thing happened your insurance maynot cover you.

Bill