What you primarily need in a use like this is a sound absorber. Funky wedge surfaces (like in your second link) are about stopping reflected sound, and not significant here.
Underlay is a genuinly good option and new underlay will be demonstrably better, so off-cuts are good.
Assuming you have an external canister under your tank, even sitting that on a foam pad (or off cut of carpet underlay) should make a noticable difference. Most of your noise will probably be low frequencies and come from being transmitted (via direct physical contanct) into the structure of the cabinet (which then acts like the body of a guitar), and the pad will act as an isolating dmaper.
Try it. If you can get a free off cut, and if it doesn't work, throw it away and you've lost nothing.