pinMode(somePin, INPUT) != off

I’m pretty new to physical computing, but as a high school teacher looking to start electronics and robotics classes, I hope to get up to speed asap.
As a first project I built a 5x24 led display, with the idea being to run the whole thing on an arduino. This is an idea i got from nerd kits video (each column pin has 2 rows of diodes with opposite bias). the nerd kit uses the same chip, but not an arduino. The code they give is in C, of course, but it’s pretty complex, almost unreadable to me, so I decided to try to write a scroller from scratch in the IDE. The way this beast is wired, I need 3 states for the pins, HIGH, LOW, and “off”. My question is, what’s off on a digital pin? I am setting pins to input to turn them off at the moment, put high impedance != off and I found I have to set the pin LOW before setting it to INPUT or I get low voltages on the pin (just enough to light the diode). Is using digital pins this way a good idea, is it healthy for my poor little nano? Is there some other way to do this?

Setting the pin to an Input is what you want, making this a “Tristate” or charlieplexed arrangement.

Why you are seeing voltage is because when you digitalWrite(input_pin, HIGH) you are enabling an internal 20K pullup resistor.