The Subtank will work on the iStick 20, but the 20 doesn't make enough power to go subohm, so you'll want to run the higher value coils. As to the burnt taste on the Nauti, it could be several things. What watts are you running? The Nauti does its best between about 8 and 13 watts. Higher and the tank is overpowered, burning wicks and coils. And some juices are really gunky. Two days seems short, but it depends on your juice and how much you vape. Tell me more, what juice are you using and how hot are you running the Nauti?