Nice pond Rick. By your pic, I can't tell how you have your pond set up, and that really doesn't matter, but in my experience, the more water volume you can move (usually the whole pond should circulate through pump and filter in an hour) the better. The amount of water that you have trickling into your goldfish pond may be a little on the low side which is why Annette talked about using a pump with a bubbler or aerator to get more oxygen in the water. She was talking about your QT tank when introducing new fish, but this would help your pond now too. It's too late for a QT now you have already put your new fish into your pond. Unless you have really expensive goldfish, I don't know that I would spend a lot of money in treating them, except for medicated food. Getting your water quality up to snuff would be first on my list. Then everything else would probably correct itself.
When a waterfall drops into the water with a lot of force it creates bubbles in the water (oxygen)which the fish really need in this hot weather. The warmer the water, the less oxygen it has in it.
Sounds like you have a lot of problems going on at once. Don't be disheartened, things will come around and all will be well. Just don't panic, be patient. I know it's hard when you are losing fish. I agree that the #1 problem is your local pond guy didn't give you good advice from the start, but he may just not know.
It's late in the season for this now, but the more plants you can get into your pond via thru a bio-filter, vege filter, or oxygenators such as hornwort and anacharis, top pond coverage like hyacinth, lettuce and lilies, the better it will be for your fish also. Their roots filter out impurities and help improve water quality. You could plan for next spring now as the season winds down--with not a lot of expense.
Oops, I see you said you do have a bio-filter--do you have it shut off temporarily?