Can a broken water heater kill an Arowana?

[Bderick67]

Thats my point, all fish tanks are grounded as well, unless somehow, someone has one that free floats..

No all fish tanks are not grounded. Glass and acrylic are insulators(non conductive) so ungrounded. The tank may become grounded through a faulty piece of equipment, or intentionally with a ground probe. Worse case scenario a person sticking their hand in the tank may provide the path to ground.