Sorry this plant Marimo Moss Balls is now banned in the USA. It is not legally for sale anywhere as of Mar 2021.
Marino balls develop their characteristically round shape because water currents push them around the bottom of lake beds. This algae is free floating so you don’t need a substrate in your tank. As it is photosynthetic, it will float or sink in response to light. The oxygen they produce makes them lighter so they rise to the top. To make them sink again, just squeeze the air out. It became a problem for me in tanks with plenty of Co2 and strong light. I finally decided to attach them to rocks and roots.