[DiverDude]

A grain of salt (pardon the pun) is called for here.

120VAC from a wall plug can certainly kill you -and as it has been pointed out, it takes mere milliamps (thousandths of an amp) to cause your heart to fibrilate. However, this in only even remotely likely in certain circumstances where the current path is across the heart (like if you had your left hand in a live tank and your right hand on a low impedance path to ground -like a lightswitch).

If you have a grounding probe (that is properly grounded) in your tank and there is an exposed wire in the tank then:

1) If there is enough conductivity between the live wire and the ground probe (like a dead short) then the circuit breaker will blow. That's when they're designed for; to protect property and prevent fires, etc. (not save your life).

2) If the conductivity is high enough so that not enough current flows to flip the breaker -OR- if you do not have a ground probe, then you potentially have a body of water at anywhere up to 120 Volts. In this case, if you put your hand in the water and present a path to ground, then, yes, you can get a shock. However, unless, you are barefoot, standing in a puddle of water that is in contact with something that is grounded, then while you may feel something, it is unlikely that you will be injured.

In fact, the most likely injury you would suffer would be from yanking your hand out and hitting it on your light or something.

Electricity always follows the path of least resistance (it's lazy, just like us) so if the ground probe is present it will present a low resistance path to ground and you will conduct very little by comparison -unless of course you're still standing in that puddle in which case you could be in trouble.

You can drown in less than an inch of water too; you just need to be face down, covering both your nose and mouth -oh, and you'll need to be unconscious at the same time -all possible but not very probable. You stand a far better chance of winning the 6/49 than being killed by your aquarium.

Am I saying this is all fear mongering ? No. Never compromise on safety and GFCI's and ground probes make sense and are inexpensive anyhow. But if you don't have these you are not necessarily 'just waiting to die' either.