Thinking about how I would proceed, I would remove the INPUT_PULLUP directive and set the pins as INPUT. I cannot see why your code needs to set the pins HIGH (lines 15 - 18) because it is the same thing as setting the internal pullups on. I would advise we turn them off. To do that remove lines 15 - 18 and change lines 9 - 13 to pinMode(firstLevel, INPUT); would also suggest we use an external pull down value that will give us a bit more current in the water - while this will increase ionisation somewhat, it will also reduce any effect during the microprocessor read cycle. I would try either 10k or 4k7 pull downs.
(An alternative approach would be to use the internal pullups and discard the external pull down, then measure from ground to pin rather than vcc to pin .