While building everything, I hadn't thought about the voltage spike that occurs when shutting off the solenoid valves, what was causing bad sensor readings, and sometimes even a restart of the Arduino. Because of that, I had to replace all the solenoid blocks of the solenoid valves with another sort of solenoid block which has screw terminals and space for installing a 1N4007 diode.

So, I replaced all the solenoid blocks and also changed the wiring so that the wires of the valves no longer cross the wires of the sensors which solved the problem.

I also replaced the brass valves that I intended to use for switching the ink because getting rid of the voltage spikes is essential for operation. 

The new valves are made of aluminum and I guess that they can also handle the ink that I want to use. For now, I'm using just water and later I want to try isopropanol and acetone mixed with some additives.