Adding more sand substrate

I 'm having trouble rooting plants to the bottom of my tank. Even the ones
which have weighting attached eventually end up floating round the tank. So
I was thinking of adding more sand (I don't have gravel), as most areas of
my tank only have about 1/2 inch of sand at most, many areas have much less.

Once I have cleaned the sand, is it OK to put it in the tank while the fish
are in, or should I remove them, add the sand, let it settle, then add the
fish?

Thanks

Andrew