Keeping our ponds clean and healthy uses a multi-faceted approach. This includes setting the pond up right with good filtration, etc but often overlooked is regular maintenance of our pond plants.
Part of the cycle of water lilies and other aquatic plants includes foliage turning yellow then brown and then rotting off. This is normal as long as new foliage is also emerging. To help reduce the amount of sludge build-up in your pond, remove dying foliage, including spent flowers, from plants before it has a chance to fall into the water and decay. Plant matter left to decay increases the turbidity of the water as well as provides a nutrient source for algae.
Removing flowers that have finished blooming and have starting sinking into the water also allows the plant to use more energy producing new flowers and more lush foliage.
While cutting tools exist, the approach we most often use on waterlilies is to loosely grab the stem right below the pad and run our hand down as close to the base as can be reached then use a thumbnail to clip the stem and pull the old pad off.
Basic routine maintenance like this goes a long way towards keeping a healthy pond.