Either a number of filter pads kept in a mesh bag in a high flow part of the sump (I have some in continuously since I’m actively sticking) fir a month or two. Just put one in your hob before you put fish in QT.
Or use a new (or newly cleaned) filter, turn off hob, squirt a bunch of Biospira in the filter, let sit fir 1 hour, then turn on to circulate while you go buy your fish
Rocks etc aren’t recommended if you plan to run copper since it absorb and leaches out at a later time. It also makes keeping the level of copper harder as you have to account for the absorption