You'll need a GFCI outlet to feed your power, and you'll need a way to ground your grounding probe.
Yeah, those older houses just ran 2 wire cables for all the wiring with no ground wire. So likely you don't have anywhere to connect your ground probe. You can't just connect your ground probe to the white neutral wire, that will just bring back anyway wayward current in your tank back to the circuit and the GFCI will still think the circuit is balanced and not trip. You need a ground, if possible it'd be best to rewire circuit you are using with typical 3 wire cable (hot, neutral, ground). But I acknowledge that might be A LOT of work especially if the circuit you are using goes to a lot of places and outlets in your home. You could just run a dedicated ground wire to your tank location to your electrical box ground i suppose. Full disclosure I'm not an electrical code expert, so you should check with a licensed electrician to make sure you follow code (it exists for a reason). What i said is functionally correct, but please make sure you follow code is all i'm saying.