This tank is a Fluval Edge and it is meant to be filled so that the water is in contact with the top glass. If you want floating plants, those typically used won't work. Plants such as frogbit, water sprite, water lettuce and salvinia stick up out of the water. The only way to use these is to have the water level lower so there is an air space over the water.
Other plants can be used as floating plants though they can also be planted in the substrate - eg hornwort and elodea. With that brancy wood, you also have the option to wing hornwort stem round the branches and leave the growing end of the stem to float on the surface. But with that tiny opening in the Edge, pruning hornwort to keep it under control might not be easy.