What 'slots' does is to cycle through each cathode every so often - and this can reduce the build-up of material and lengthen the life of the tubes. And that's something you're going to want to do!
The nixie clock I built in 2006 is still going today and looks like the day I turned it on - it cycles the digits every 60 seconds - as the minute rolls over, the numbers all cycle for a few seconds. Looks cool and keeps my tubes in top condition.
I'll work on this next :)
Ok! Back to the code!
The code is a bit messy - but what code isn't?
How this all works -
The OneWire DS18B20 is first declared and set to reside on Pin 19 (this is A1 on the Pro Micro)
Next is the declaration of the three digits for the display: