Grounding probes take that voltage in your tank and turn it into electrical current, which flows through all the inhabitants of your tank. A standing voltage is not detrimental, flowing current is.
For instance, when you touch a doorknob and feel static electricity, it's because a voltage has been built up in your body. You don't feel that voltage until you touch that doorknob (in essence, grounding yourself). The voltage in your body discharges to the doorknob (which has no voltage) because the voltage is trying to balance itself between your body and the doorknob. This balancing results in current, which Is the shock you feel.
GAC is granular activated carbon, which has been proven to cause HLLE.
Selcon is a vitamin. HLLE has been linked to nutritional deficiency.