The idea is the ground probe makes a better ground then you. You know how you always here electricty follows the path of less resistance. Your just making sure thats not you lol.
A GFCI on the other hand just trips if it detects current on the ground. Thats why they call them Ground Fault Circuit interrupters. With out a GFCI its very easy for a power strip or any other peice of equipment to catch fire. A breaker won't trip just cause you have a short.. Infact you can have a powerstrip on fire and shooting sparks.. As long as its not pulling more then what the breaker is rated for it wont trip.. on the other hand a GFCI will trip instantly. They also are suppose to trip before a fatal shock is given, IE if you drop your radio in the tub. You will get shocked, but you should live.. With out the GFCI your pretty much sitting on old sparky..
keep in mind some GFCI/ballasts dont get along.. I think its a good idea to have two circuits or in this case two differen't GFCI external plugs.. Have your lights and maybe your heater on one and your pump on another.. If you have more then one pump split them up on the Two GFCI's. That way if one trips you still have water circulation.. Most people that don't use GFCI's cause they are scared they will trip for no reason and kill the tank.. BTW dont plug the second GFCI into the first :rolleye1: would kind of defeat the purpose.
Will