Maybe, but the important thing here is the GFCI. It won't make it any safer for you to add the ground probe, as the GFCI by itself ensures that no dangerous current flows through anything - either you, or anything else. The probe doesn't hurt, as long as you don't use it as an excuse for not doing the GFCI.
I guess one downside is that the ground probe may increase the likelihood that the GFCI will trip when you aren't around, which could leave the tank without heat or filtering long-term: is that better, or is it better that they switch off when you're standing there and put your hand in the water? I dunno.
BTW, a GFCI is called an "RCD" in the UK and Oz.
