I don't see how that would keep the tank from slipping around, as the paracord would still be able to slip around your waist.
I'm assuming you have one of the 4+1 (or 2+2+1) packs that slip over a belt, so my first question is what kind of belt are you using? The cheap one that came with it, or an upgraded one? Do you have it tight to your body at your waist, or loose at your hips (where your jeans would sit)?
What you want is to increase the friction between the pack and the belt. The best way I know how to do that is to get a belt that is just as wide as the slot for the belt and is nice and firm. I use a velcro lift belt kind of like this one:
I think I got it for 10 bucks at a sporting good store, It improved the functionality of my packs a lot.
Barring that, you could get a large safety pin and just pin it to your belt.