It has to take a finite time for CO2 in solution to move a certain distance in the water, from higher concentration to lower concentration. That would support there being a gradient of concentration from top to bottom. Also, CO2 is in the tank so the plants can absorb it as a nutrient. Immediately around a plant leaf there has to be a gradient in CO2 concentration, low at the leaf surface and higher farther away. Fast growing plants must absorb nearly as much CO2 as we add - a few "bubbles" per miinute. I have no idea what the speed of dissolved CO2 in the water is, as it moves from high to low concentration areas. If it is sonic velocity, the concentration would have to be almost totally uniform throughout the water, but if it is inches per minute, there have to be large gradients in concentration.