RE: Aquatic Plants Digest V2 #379

>The voltage of _anything_ used in your tank is more or less irrelevant. 

Disagree, I = V/R as you said, therefore changing V will have an effect on I,
the current flowing (it also depends on the resistance), but for a given reistance R,
the higher the voltage, the more current will flow, so a voltage of 240V in your tank
is far more likely to affect you than 24V.

 >THE CURRENT is what shocks you and/or your fish.

Why will your fish get shocked? If you consider the electical "leak" in your tank
to be a point source, unless the tank is an old one with a metal frame, the glass
of the tank will isolate the body of water from ground, and the entire body of water
will float up to the potential of the leak. Only when you put an object into the water
that is grounded, will a potential field be set up. 

Assuming that the object is small, the EMF field set up will look like spheres
surrounding both point sources, meeting as a plane field equidistant between the two. 
Assuming that the two sources are not close together, the number of potential lines
that each fish can cross (this will depend on the length of the fish) will determine
the potential voltage that can be generated within the fish. The current that could
flow through them would then also depend on the internal resistance of the fish, 
assuming a similar resistance to ourselves, it would not be great enough to cause
any trouble with the mains voltage we get in our houses, ie 120-250VAC. 

>Folks sell nice neat electrodes that are supposed to conduct any stray
>voltage directly to ground.  Nice idea, but it is pretty much useless.

Not so useless if you have RCD/GFP (Residual Current Device/Ground Fault
Protection) fitted, it will pick up the fault long before you put your hand into 
the tank. These devices will shutdown with only a couple of milliamps flowing.

Assuming that you do _not_ have any protection fitted, a ground probe in the 
water connected to mains earth/ground will set up the potential field as
stated before, and so the potential at the water's surface is likely to be
much lower than with no probe at all. This does not mean that you will not
be shocked, only that you are less likely to be killed.

>Well, they just provide a path from the current to the ground.
 >If the leakage is from the other end of the tank, everything in the tank
>between the electrode and the leakage source is getting shocked.

Placing the ground probe as close to any electical object in the water will 
greatly improve matters, as the EMF field in the water will greatly reduce,
and so you possibly will not even be shocked as the field will be to weak,
since the majority of the field will be concentrated between the leak and
the nearby probe.

>Frankly, I think
>grounding electrodes are a lot like those water softening magnets.

I disagree, I think that the difference between getting shocked and getting
killed is very important. The difference is that this effect can be verified
experimentally, whereas your water softening magnets cannot.

Setup a tank with a voltage source at one point, a ground at another, and
use a high impedence multimeter with one probe connected to the ground
and probe the water, you will be able to plot the field by taking measurements
in a grid like pattern accross the surface of the tank.

Move the ground probe close to the voltage source, and repeat the test. You
will notice the lowering of the EMF field on the surface.