Please help

Hello,
I never posted before, but I am desperate for advice. I have had a pond with goldfish for several years with very few fish dying. I have a pond with about 15-20 goldfish that appear to be going to the top for air. I recently went on vacation and before I did, we won like 4 goldfish at a carnival and I put them in pond. I hired somebody to feed fish while I was gone, may have fed too much, and when I got home I cleaned out all food and changed water and put in dechlorinator, I haven't feed since because they keep dying and I don't know what to do. I don't know if this is enough info but if anybody can provided advice or help me out on what I should do next, please help!! Could overfeeding for a week kill all these fish to where they are going to the surface for air even though filter is running strong and done all the changes as stated above? I am at a loss what to do next, I am going to fish store in a bit to buy a test kit and wondering if I should get antifungal or is there anything else I should get?
Please help if you can, thanks.

Thanks for the help, I just did all the tests and no ammonia, ph level, nitrite are all normal so it looks like my problem is elseware. I do notice the scales of the fish coming off so I also bought some sea salt, but I don't know if I put enough for an approx. 500 gallon pond, it says a table spoon for every 5 gallons and I only put in like 100 gallons worth just because I am not sure if this will shock the fish. They are all still breathing really hard gasping for oxygen, and I have the filter going, I will put the waterfall on too. The guy at Petsmart wasn't much more help. Any other ideas.

still needing help

and the water temp is in high 70s

picture of my fish

I just tried to attach some pics, hope I did it right.