When inspecting your pond, you should be looking for any plants that are taking over too much space or suffocating others, and get rid of the excess in order to re-establish a natural balance.
Water lilies may become quite overwhelming with very large leaves and no flowers; at this time of the year water lilies are dormant. Another plant, which can become very big and abundant is the Pontederia cordata or Pickerel Weed, it has beautiful flowers, but unfortunately the plant can reach quite a big size.
This is the best time to remove all unwanted plants from your pond, as most of them are dormant and easily removed. If you see that some plants are simply taking up to much space, or are not blossoming as they used to, pull them out and replace them with new plants.
Once you eradicate all unwanted plants, your pond may look rather empty, but water plants flourish quickly, simply visit your water garden nursery and replant them in your pond, it will soon find its luscious aspect.
You may choose between various oxygenating plants such as Iris laevigata Variegata, Equisetum hyemale (horsetail), Caltha palustris (Double Marsh Marigold) and Ceratophyllum demersum, which is a floating oxygenating plant or even simply grass.