Convert 8 bit Parallel output from ULN2003 to serial without any Microcontroller
The simplest 'no MCU' solution would be analogue.

Have an octal (or hex as you only want 6 electrodes) buffer at the tank, either the ULN2003, or perhaps better still a CMOS type like an HC7404, with a simple resistor to rail at the outputs. As each output goes valid, it draws more current from the rail.

The receiving end uses a current monitor resistor and a bargraph voltage measurement IC (like an LM3914) to monitor the current drawn.

The advantage of CMOS is that you can use very low currents at your tank probes to minimize electrolytic corrosion, perhaps 1 M&Omega; resistors, or more.

I'd tend to use stainless steel as an electrode material, relatively inert, and cheap to replace when they finally rot.