Too long for a comment but not really a proper answer, so some advice.
I’ve heard of "ammonia spikes" which seems to tally with your story. I’ve had them myself: beautiful fantail, white tail, red lines appeared one morning but all readings were in the safe limits. No other fish affected. The blood streaks didn’t really disappear, either, although the fish lived a reasonably long life and didn’t seem bothered by them ever.
That said, the streaking I had didn’t look anywhere as bad as this (it was limited to a few lines on the huge fantail tail and some on the pectoral fins). Also, going by your photo, your fish look stressed: the dorsal fin is down and the other fins are clamped. It could be that they’re stressing about the camera just at the point of the photo being taken or the water change. But they don’t look happy in the photo, even without the blood streaks.
There’s one other thing it could be. Bacterial infections can cause blood streaking like this but the only time I’ve seen it, it was on the belly and the fish had had dropsy for some time before that. Your fish don’t have dropsy but do look out for it. Dropsy is bad.
Some things to check:
- Post numbers here for your chemistry. There are some experienced fish keepers on here and posting numbers is usually the first thing requested.
- Check your pH. I had issues with a decreasing pH (soft water area). Didn’t realise for years and my resident fish seemed to adapt as it crept slowly down. All fixed with a coral gravel buffer.
- How is the water in your area? Any chance they’re changed the chemistry at the water treatment plant? When was your last water change before this on? Do you have to worry about chloramine?
- Have you dosed with stress coat?
If it’s all of your fish, it’s likely to be a water issue. Make sure your strips are in date and consider taking a sample of water to your local fish store for them to test. Some fish stores even supply water, too. Make sure there’s nothing gumming up your filter and check your "stock" levels. Goldfish are big and dirty. I have 3 in 180 litre tank and I would not consider more (even though 2 of them are still tiny). Overstocking just makes for more work and I’m lazy. If your fish are still quite small, the 56l tank might be plenty big for now.
If it’s just one of your fish, see if you can see anything else on the fish. Are it’s gills a good colour? I must say I can’t see anything else wrong with it. Is it eating and pooping ok? Keep an eye on the other fish.
Lastly, goldfish do change colour sometimes. One of the photos makes this look bad, but the others not so much. I think it’s unlikely given the actual colour change but not completely impossible.