Rectifying diode for boost converter in nixie clock
D5 is part of a high frequency boost circuit formed around the MOSFET Q7 and the inductor L1. together they dump energy into capacitor C4 and they will use high switching frequencies possibly circa 100 kHz. Generic boost converter: -



Your boost converter featuring D5: -



This means that you must use a fast diode or the extended reverse recovery time will significantly reduce your high voltage rail and/or efficiency. Reverse recovery time means that when a diode goes from forward to reverse bias, current will flow against the grain for a short period of time and for a 1N4007 (for instance) this can be as long as 30 us. 30 us would severely disrupt the working of your boost regulator.

Fast diodes are in the low tens of nano seconds with a diode like the BAS16 being about 5 ns (for example). Nixie tubes are cool BTW!