I'm wondering if it is possible to have too many plants in a pond? I know that it is important to have 75 to 85% surface cover to keep water from heating and algae from growing. This morning I took a good look at one pond and decided it had about 100% surface cover with Parrot Feather. The original plants are rooted in a centrally located pot of soil (about one foot in diameter) with the upper edge just below surface level. The stems cover the entire surface 6 foot diameter circle), and stems and roots seem to almost reach the bottom of the pond (2 feet deep). The only open surface area is small and is kept open by the discharge current from the submerged filter. If a fish were to die it couldn't possibly float to the surface except in that one small area. I thought this would be good cover for any goldfish fry but am beginning to wonder. The only other plants are a small group (bound to get larger) of water hyacinth and a mix of azolla and giant salvinia floating on the surface between the parrot feather stems. I think that when the biological filter is really working well and pulling all the nitrogen out of the water the parrots feather will begin and fail and that could spell trouble. Last year the pond surface was covered with lily pads but the plants only covered the surface and didn't fill the pond. Not a single goldfish fry survived, and I know they spawned and the eggs were fertile. I suspect the single, female dwarf gourami that survived the winter had a lot to do with that. Surprisingly the same gourami also survived this past winter but several weeks ago got caught in the net over the pond to keep out the cats and died.
Milo

I dun know about pond for I am a tank keeper..

but in theory, I think it will be better if there are some contact air contact with the pond water.. this is to allow any undesirable gaseous to be released (nitrogen that will become nitric acid and carbon dioxide that introduce carbonate acid).. air contact also allow the water to absorb more oxygen from the surface too.. unless you have a waterfall somewhere that allow the water to have some air contact before allowing it to flow back into the pond.. maybe a small fountain or water fall that flows down a series of rock (coral rocks or lime stone) sounds good as it will also buffer the PH..

I have about 75% of my pond covered with plants. You need to let some sunlight through.

Parrot Feather in particular can be a pest, and the plant is much bigger than what you see on the surface. Its long roots form densely growing patches under water, and I lost one of my western painted turtles because she got tangled up in the roots and couldn't make it to the surface anymore, therefore suffocated. I would cut the plant down on a regular basis....