That's not true at all. If there are say 10ppm of nitrate in the well water, the only thing that affects is frequency of water changes. You will have to do W/C more often, because your starting point of nitrate is 10ppm. It's just logical. How is using water that already has nitrates in it harmful? Fish waste is converted into nitrate by beneficial bacteria in your tank every day. It's safe for most freshwater fish to be at 20ppm of nitrate or so. OP: Did you have a filter on your outdoor fountain? If so you should bring that used filter media inside and put it in the tank with the goldfish. For the injury you could add Melafix to the water (as per package instructions) and that ought to help heal it.
I never said anywhere not to do water changes because of that. What I'm trying to explain is that water conditioner like prime will completely get rid of all of these elements. For example:
Say you have 20 ppm of nitrates. You do a fifty percent water change with 10 ppm of nitrates in your water. Where are you? Where you started. Okay so that's not harmful. What about in a larger scale. ---
There's 100 ppm of nitrates and you want to save your fish. You do a 50% to get it down with fifty, and add 10 ppm back in. 60 ppm of nitrates. Do another fifty, then you have 40 ppm. Water gets low, and you too it off. 50 ppm. 50% water change, down to 35. The cycle continues. Eventually you'll never be able to get below 10 ppm. Now take in account the amazingly high bioload of a goldfish. Over time water changes will be temporary fixes for nitrates. Just a capful of prime can stop that lol.
And also, nitrates can be deadly for fish. You'd be surprised. It's against the law for high nitrates to be in human drinking water. Fish live in this water, so it's even more serious. Nothing against you at all lol I'm ranting about some common misconceptions in aquaria in general. Just because fish live in it, does not mean it is healthy. Think of it like this (this is a pretty distant analogy, but still definitely explaining the point):
somebody eats pure sugar from a packet and it doesn't affect them. Sure it's fine. Another packet, well that's okay. Etc etc etc. eventually you will die (it will take a lot, but still). just because you're alive does not mean you did not suffer. You get headaches, dizziness, and other symptoms. Fish can't tell us these so called symptoms. When nitrates are high there's nothing they can do. Although they live, it is not healthy. Of course it's MUCH healthier than ammonia and nitrites.
anyways i do not mean to come off as rude at all so do not take me the wrong way at all. Just trying to state that buildup of anything (okay buildup of money is not lol) is a bad thing. Some people may not work like you may to do more water changes to lower nitrates. In general it just causes more work.
Anyways, back on subject. As somebody already stated the outside temperature could hamper bringing in the filter media from outside. There are different strands of the nitrifying bacteria in different temps if I'm not mistaken.