Best way is to run a sponge filter in the main tank, then when you use the QT
, put the sponge filter in it. Then disinfect the sponge filter after the QT
is over, and put it back in the main tank.
Or take spare established biomedia or floss from the main tank's filter, and put it in the QT
filter. After the QT
is over, discard the biomedia or floss.
I would skip the sand, you would have to disinfect it or discard it if you ended up treating a new or sick fish in it.