If you read the Regs you will find the low voltage is anything exceeding 50VAC or 120V ripple free DC but not exceeding 1000VAC or 1500V ripple free DC
now "extra low voltage" is anything not exceeding 50VAC or 120V ripple free DC.
now you can see the even when a electrical engineer say "Extra low voltage" Is can still do damage to you.
And suphew is right most older houses wont think twice about passing 10,20,30,40 amps through yo to ground, it only takes in the order of 100mA (0.1 amp) across your heart to stop it.
It is my strong recommendation that you (fish keepers in general) find out if your house is RCDed and if it is not, get a electrician to install one in your board or go out and buy a RCD to plug into your power socket and run your tank off it.