do you mean, 'how to keep plants from floating out of a waterfall tank' or just how to grow them?

how to keep them from floating out of the waterfall tank

You might use a length of the plastic gutter cover, affixed across the weir area to contain them or just start your plants in pots raised on cinder blocks to raise them to the correct level. They will grow rapidly and then once they have larger root masses, they will stay in the tank without the pot.

You will have to monitor for the water being diverted out of your pond by the roots. That happened to me. Those plants really like fish poo water and make a lot of roots. It is worth it to have the beautiful plants. Impatiens love being in a stock tank like that-lots of blooms. So do all the mints but you will have to watch out for the roots to not get away from you.