I'd say just remove the older ones. Since the leaves grow from the center of the plant, the peripheral leaves are the oldest and worst looking. The longer they are on the plant, the more likely they are to get covered with spot algae, disintegrate at the tips, turn brown, etc. Once they start looking tired, cut them off or pull them off at the base. One caution for pulling though: over time, you can start to pull the plant up and end up with a mound effect. If you cut the leaves, the stem ends will rot away and can be easily pulled off.