That is awesome, dont worry about the fish, they have survived longer than humans on this earth. Nothing special needed for them, even over winter. Im not saying dont but I never worried about icing over. They go into a semi hibernation. I remember walking on the ice and seeing them swimming below it.
By buying what the industry calls "feeder" goldfish you are doing them a favor at a chance of survival. They all won't survive but they are living more like a free fish now. I wouldn't get crazy about adding chemicals. Think about it like oil additives most are useless but in this case nature has your back and you may not be aware but all the plants you put in the pong already added billions of good bacteria, as does every left and peace of dirt that falls in.
Very cool, good luck.