Feeder fish in pond?

Question:

Hi, I recently completed my first pond, around 300 gal. I have a Supreme Pondmaster 1000 filter on the bottom with a Pondmaster 250 gph pump connected to it. My water temp is 54 degrees and water clarity is good. I have about 2/3 of the surface covered with water hyacinth. I filled the pond with city water and did not treat it chemically, having heard that the chlorine would disapate out of the water my itself is a couple days. 2-3 days after filling I bought around 50 “feeder” gold fish from the pet store and released them into the pond. I’ve been feeding them flake type goldfish food twice a day. Iv’e had about 15 fish die in the two weeks since they were introduced. They become listless, then inactive, then dead. The other fish are vigorous and active. My question: is a certain amount of die off normal from that kind of massed produced “feeder” fish or is the die off a sign of disease or chlorine poisoning. Is it absoulutely necessary to treat city water with a neutralizing chemical? I’ve been putting replacement water in buckets, letting it sit for several days, and then adding 10 gal. at a time to offset evaporation. Another question: I run the pump for a few hours every 2 or 3 days. I don’t know how much is optimum. The water clarity is not a problem but what about the oxygen requirement for the fish? I’m trying to emulate a natural spring so the water flows up from the pipe outlet like a “spring” but does not flow over a waterfall or up through a fountain. Is this a problem or does moving water by itself mix with air to oxygenate the water. Plus, do the water hyacinth oxygenate the water. I had a friend who had a rudimenty pond with no pump and lots of plants and algae and the fish did just fine. I look forward to your input, thank you, Neil