It is mostly full I would say, but only because I planted it to the amount I wanted it, knowing the plants wouldn't grow fast. So it is full because I planted it not because the plants filled in. If that makes sense...could be filtration. How does filtration directly affect algae? Would you suggest changing from 50/50 mechanical to bio media to 100% bio media?
I will speak on this, even though you are asking Frank, I have done many low tech tanks without hard to grow plants (HC for example) with no CO2.
The first key thing is having the right amount of light. It may or may not be low light. I like CFL bulbs on nano tanks because you can go from low light to high light by swapping bulbs. I get two sets of bulbs, starting with the lower and if I don't see good color I up the light (more important than growth to me because good color will get good growth). Good color is relative here because you won't get crazy rich colors with the amount of light that is reasonable. You saw the plants I sent you, that wasn't great color, even for low tech but they all had color.
Filtration, IMO, isn't as significant but you want good filtration. More good bacteria, more good happens in the tank. However, I have had plenty of low tech tanks that didn't have great filtration that worked great.
Watch what happens after water changes. If you get algae afterwards, do smaller, or less often, or smaller less often. That changes the CO2 level. Even though you are not injecting CO2, their is CO2 in the water. Large water changes lead to inconstant CO2 levels.