This is easily fixed
@Christian , no need to try and play the isolation game and has nothing to do with transient interference from nearby wires or how the Atlas Scientific pH module works.
The problem is the way power supply is setup for the Hydros controller, unfortunately the controllers ground, AC earth and tank water are isolated from one another, when you add the heater element it has the outside case connected to earth for safety. Unfortunately this means there is now an alternative path to "ground" (earth in this case) that changes the reference "ground" for the pH probe. This can be made worse from devices like Salinity and ORP probes as well one of the newest sources of interference being the conductivity sensor in roller mats use to detect the water level.
It can be easily remedied in a couple of ways, the simplest is to add a grounding probe to your sump somewhere near the pH probe (underwater obviously), and connect this grounding probe wire to the ground (negative) on the controller the pH probe is connected too (you can use any ground point on the controller, including a sense/sensor/IO/power port). Just remember to recalibrate you probes after connecting the ground reference.
This will resolve your problem assuming you don't have any devices with the L & N connected the wrong way around internally (like in some cheap usb charges). All saltwater tanks should have a grounding probe connected to the Earth on an AC socket for safety, you can also connect this same probe to your ground on your controller for simplicity.
