Leaving a gap in the bottom of the divider will let water and critters go from one side to the other. I've thought about doing something like this with a
betta... You would almost need to have a tall tank so that most of the bottom would be the aquarium part with lots of swimming room. You could use a deep plastic tub to hold dirt for the plants/land part and either put long legs on it (like Roman columns) or have it hanging from the top framework. Fill the tank about half full of water and the vivarium part would be the top half. If the tub with the dirt is only about half the diameter of the tank, then that would leave plenty of surface area for the fishies to come to the top to be fed and for the filter and heater to work.