Hi. I am constructing Greenhouse Automation with Arduino (mega). The project is nearly complete.

Main part of the project is the control unit where I have Arduino, PCB (before breadboard), LCD 16x2, breadboard power supply and 5v relays. This unit is connected to sensors dht11 and 2 soil moisture sensors. Relays will be controlling 4 circuits:

I tested my control unit several times without connecting it to the greenhouse and actuators. Required relays are activated when needed, depending from temperature, etc.. Generally no issues. Today I mounted the unit in greenhouse and tested again without connecting the fan, solenoids, or actuator. Still the same - everything works fine. Now the problem is: I turned on and off the fan (230V) using switch on a fused spur (13A) which is in the same mains radial circuit as sockets where breadboard power supply for my unit is connected. When I turn off this fan, LCD starts to display strange characters, like there was some power interference (I don't know how to name this anomaly). Then, I turn Arduino quickly off. It happens most of times when I turn the fan off but it happened also once when I turned it on. Does anybody knows what is causing it? Is LCD (16x2) so sensitive to power fluctuations or what? Fan is from Amazon (likely Chinese -38W) but as I say it is still not even connected to relays. Thanks very much for help.