The size of the bloom is not due to food. Some have large flowers
some small. You have short, medium and tall Cannas. There are green leaves, stripe leaves (green, yellow, white), Variegated leaves, Red leaves and many variation inbetween.
If you look at my photo albun you will see I have several different cannas.
I grow most of mine in pots because they are not hardy in my zone
except one. It is the one with small red flowers
with tall green leaves. It is the only Canna I do not like. LOL!!!
Since you are renting I would buy plant
some flowering seeds
that have a lot of red blooms and it would accent the canna when it blooms.