Tricky question about how Velvet treatment failed

Let me preface this by pointing out I'm not a beginner, as many people seem to assume whenever one asks any question on this forum.

Anywho
I recently moved several rescue goldfish (who were already stunted from previous owners, though I managed to grow to 6+ inches. I had the oldest for many years, and the newest about 6 months. The newest was actually 2 young(?) koi I was hoping to grow out and rehome. There was also a random Odessa barb in there (long story)) into a "new" 72 bowfront (Bought used, had sat empty with the previous owner for a year, and sat empty with me for a year) which had been cleaned with vinegar (and rinsed thoroughly!) prior to moving the goldies in. They had a 90 gallon aqueon HOB that they had had for years on their previous tank, and I did not observe any hiccups with the nitrogen cycle (No spike of ammonia or nitrite, only my expect/normal levels of nitrate).
Several weeks after the move to the new tank, I noticed velvet on one of my goldfish (On August 18th). I've never *personally* had velvet show up in my tanks, but contact one of my fishkeeping friends who has it pop up frequently. She recommended Ick Guard (for its active ingredients) - for her, this clears it up within two days. I followed the instructions to a T (I had pure filter floss in my filter, no carbon), and it did absolutely nothing. I did a large water change, then redosed. Nothing again. Chatted with her again, and decided to try Ich-X for several days (For the malachite green in it). No change, except the first death was on August 25th. Added Coppersafe a few days later, along with continuing Ich-X. Nothing.
The last one passed yesterday, less than 3 weeks from when I started treatment. Now the weird part, the two koi and the barb are perfectly fine. Either they never had it or it cleared up sometime during treatment - I'm not sure, I'll admit I don't recall specifically looking at them. Editing to add: I also blacked out the tank when I noticed the velvet.

So here's my question - What the heck happened? Why did none of the treatments work on the goldies? Why were the koi and barb fine?

Even if I misidentified the velvet (which I'm pretty sure I didn't, and my other fishkeeping friends agree) it was still some variety of external parasite. I know velvet, by the time one notices it, is usually quite advanced, but still.
Also, I doubt anyone will know the answer to this - but where the heck did it even come from?! It can't just magically appear but I'm stumped!

I'm absolutely heartbroken about the goldies. I had the oldest so long... and one of the others was my grandma's before she passed. I'm just incredibly frustrated and stumped by this all and was hoping someone here could offer some insight. My usually group of folks I talk with also has no idea. If it crops back up I'd like to be prepared.

 

I'm sorry about your fish I don't know much about velvet vs ich but from other members here (mostly the betta forum years back) one of our senior members recommending a cooper based medication along with a tank blackout.
Velvet can be in the gills where it goes unnoticed before it can be seen on the outside of the fish.

Obviously, do more research, wait for others who have had firsthand experience with it.
I wouldn't normally answer a thread on how to treat velvet but since you've sadly lost your fish, no lives are harmed by what I have picked up over the years.

 

I'm sorry about your fish I don't know much about velvet vs ich but from other members here (mostly the betta forum years back) one of our senior members recommending a cooper based medication along with a tank blackout.
Velvet can be in the gills where it goes unnoticed before it can be seen on the outside of the fish.

Obviously, do more research, wait for others who have had firsthand experience with it.
I wouldn't normally answer a thread on how to treat velvet but since you've sadly lost your fish, no lives are harmed by what I have picked up over the years.

Yeah, I did use copper. Though it is slower acting than the other meds I used, which was why I started with the faster meds (Because, as mentioned by both your comment and my initial post - once you see velvet on the outside of the fish, it's advanced, and I did not expect the meds to fail so completely). I DID however, forget to mention I blacked out the tank once I noticed the velvet! Edited the original post to add that, thanks!

I'm decently familiar with the treatments for velvet, despite my never having personally had it before. I'm more curious if anyone here knows why it didn't work. (Or why the non-goldfish were different)

Also, yes, ich and velvet are different parasites. However, they are both external parasites which means the meds for one are usually effective for both. You'll note I mentioned that I used the "Ich" medications for specific ingredients - I knew I was not treating ich. They are very different in appearance

 

It is my understanding that stunted fish likely have compromised immune systems. Perhaps this is why it took down your goldies so readily? Sorry for your loss

 

It is my understanding that stunted fish likely have compromised immune systems. Perhaps this is why it took down your goldies so readily? Sorry for your loss

Good thought! I forgot about that. My oldest goldie had pulled through a nasty fungal infection in the past, so I always thought of her as pretty hardy... That said, not all the goldies were stunted. There were 2 I'd bought new/myself - my grandma's and then a shubunkin that was too cute to pass on (Also knew it would outgrow the tank). My grandma's was the second to pass, though the shubunkin was the second to last to pass (my oldest was the last to pass).
Definitely something to consider

 

Other random ideas... The meds may have stressed the older goldfish out more than the young koi. The move to a new tank may have been more stressful to the established fish if they had been in the same home for a very long time. Maybe the coloration on some of them made the velvet harder to catch quickly? (I've never seen it in person, but I'd assume gold dust on a goldfish might blend in to some extent.) I think velvet life cycle involves them dropping to the substrate and then going back into the water column to search for hosts- maybe this targeted the goldies more if they shuffle through the bottom often more than the mid-water barb.