Building an electrolysis tank and I want to regulate a 1.23V into the tank. Any ideas how?
What you need is a "voltage regulator".  They come in all shapes, sizes, and values.  1.23V though is a fairly specific voltage, and not an easy one to find.  It's not a "standard" voltage, so the manufacturers don't tend to make that voltage.  It may be possible to find one if you look hard enough though.

However, all is not lost, because there are adjustable voltage regulators available.  These use resistors to set the output voltage, and one of the most commonly used ones, the LM317, will adjust down as low as 1.2V, just below your required voltage.

Adjusting to precisely 1.23V though is going to take some very specific resistor values, so you may find you're having search as hard for those as you would for a fixed regulator of your required value.