A ground probe does create a path for current to flow to ground..
I have never seen any proof that this actually causes a problem under normal conditions and they used the words "may" also as they have no proof/levels,etc.. to state a truth.. But it is known that certain aquatic creatures are sensitive to electrical currents to some extent..
A ground probe is not needed though and if you do want one then you should NEVER use it without GFCI..
The best protection that is needed (a must) is to ensure that all equipment that is in contact with water and exposes you to AC voltage is protected by a GFCI (RCD) device. That will ensure that when current starts to flow (your grounded arm is in the tank and you have faulty equipment) where it shouldn't that the equipment will be instantly turned off thus protecting you from a shock hazard that may injure or kill you.
The only truth to a ground probe is that if you run one in combination with a GFCI device and if an electrical failure happens to equipment on that device that the equipment will be turned off immediately when the fault developes and not only when you or something else creates the path to ground. Without a ground probe something needs to create a leakage path (your grounded hands/body,etc..) and then the GFCI will operate.
A GFCI device will not protect you from ac voltage on the other side (not the wall outlet side) of an isolation transformer like a MH light fixture/ballast,etc.. or any other situation where the voltage in the tank is isolated from the GFCI device..
Many have run ground probes with no known issues related to them and many know they are not needed at all..