As you can see, there is a small leaf, and it look healthy than other leaves. I guess maybe you right. Maybe I should remove the leaves with brown spot.It is always difficult to tell from photos, but I wold suggest this is a nutrient/light issue. As the rhizome extends (slowly) new leaves will arise from it and if they are healthy, you're probably OK. Leaves that are in any way weakened do not recover, this is true with every aquatic plant. Most of the time the plants are re-channeling nutrients to new leaves, not trying to repair damaged or dying leaves.