HELP my gold fish are dying!

i have recently added five comet goldfish to my pond-- last monday- -got them from pet store

the pond had been filled with water for several weeks before i put them in the water--

and i treated the water with declorinator before i added the fish--

i am feeding them only once a day - food that drops to bottom and they all have been eating well and swimming well--
i have added plants for O2 and for shade--

the pond is approximately 4x6 and about 2' deep

i have a plec for algae eating-- he hides underneath a plastic shelf i placed in the pond--

the fish have been doing well up until yesterday when i found one dead-- and tonite i found another - -the dead fish do not seem to have any marks on them or have they been acting strangely-- today they all ate well and swam around just fine--

i dont have a clue what the problem could be-- i am hoping someone here can help me as i feel terrible they are dying

also why i have your attention -- can i get pond plants from the wild?
there are tons of blue pickerel growing everywhere just down the road from me...
seems silly to buy it when i can just harvest a few plants????

here is photo of my pond so you have some idea of size etc--
thanks in advance for help

There are so many reasons that fish can die, I hesitate to offer any advise for fear it will be the wrong one.

However, I see that you have a spitter to help add oxygen to the water that is good.

You dont mention any type of filter ? I doubt you will be able to have fish in that pond without some type of mechanical or bio filter. Fish in a natural pond have a constant supply of fresh water moving in and "poopy" water moving out. As you fish breath and poo in that water the water quality drops.

You could do a parital water change to help. Take out about 1/2 the water, add declor and replace water. Do NOT feed every day, more food, more poo, nasty water.

If you really want to keep a couple of fish, I would strongly advise you to do a little research on bio filters (you can build your own) and buy a pond water test kit.
good luck
I know its awful to see fish dead !