Problems with Water Feature
It sounds as though the worms, etc are entering your water feature under the liner. The liner will provide excellent conditions for snails and worms to thrive.
My water feature is sealed with no gaps between the reservoir and the surrounding water capture area. Clean water cones out if the feature and runs back in.
I think your liner needs to be sealed to the reservoir to ensure nothing can fall into the water. Obviously, this maybbe tricky to carry out depending on your access to the liner and tank, but until the unit is sealed up I think you will continue to have this problem.
I believe the solution is prevention of contaminants entering the water rather than trying to clean dirty water.
It may be advisable to find a reputable landscape firm to inspect the feature and give you further advice.
I hope I have been helpful. P
lease let me know if I can be of any further assistance.