A definition: Current is defined as the movement of charge, 
whether that be electrons or ions.

> Which ions are more mobile? Perchance, is there a table giving
> mobility parameters?

	Well, really, their mobility is dependent on their charge and 
their masses. Table? Well, you don't really need one... 

> Are charged ions more mobile than say dissolved oxygen? What makes
> ions move about? I presume its more than random kinetic motion
> (Brownian motion) in the absence of current (as in a substrate).

	Dissoved O2 is not an ion. Ions move because of electrostatic 
effects and because of Brownian motion. 
	You use the word "current", but you don't really mean current. You
mean water circulation. Yes, though, you're on the right track, the ions
do move in part by diffusion. 

> The reason I ask about this is it has a bearing on how quickly
> nutrients and oxygen and CO2 diffuse through the interstitial
> water of the substrate.

	They move by regular diffusion. If they are dissolved, then they 
are really dependent on diffusion to get anywhere. Of course, some 
solutions are denser than pure water and will not necessarily diffuse out 
of the substrate very readily.
	However, to answer your question... O2 and CO2 diffuse rather 
slowly in and out of the substrate - as do other nutrients.