Strange fish death that looks like ammonia poisioning

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Aquarium Advice Newbie
Apr 28, 2022
Hey all

I recently moved into a house that came with a large outdoor pond. It contained about 12 medium sized goldfish, I brought along 3 smaller goldfish who moved in with them.

All good for two months until my smallest one suddenly died (everyone else seems fine).Symptoms were that he became lethargic and then suddenly died after a couple of days. He also had black marks (see photos).

I've been fish keeping for a while so I immediately tested the water thinking an ammonia spike, this is what I got:

ammonia 0 (This wasn't totally a bright yellow like the card but I compared to tap water and we use prime. quite sure it is 0)
nitrites 0
nitrates 10-20
ph 7.5-8

So I'm at a bit of a loss on this one. Pond has decent filtration and oxygen flow. I also only feed every two days sparingly. I ran the ammonia test so many times and it just turned up nothing.
Any thoughts? I really don't want to see my other fish get sick
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Here are the pics:



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Ammonia is the most likely cause, but if you are confident in your testing then we can exclude that.

It could be marks from a healed injury. Did you see any sign of injury before the fish died.

There is also a disease called black spot disease that pond kept fish can catch from infected snails.
I feel pretty confident on the ammonia tests given the pond is well established, we're feeding sparingly and the other fish seem happy. I tested ammonia many times (as a sanity check) with it always coming out the same as tap water (quite close to yellow and we use prime, my eyes aren't great with colour but wife agrees).

Looking at the mark more, I think you might be onto something with the injury theory. The very strong/localized nature of it leans me that way over an ammonia burn (which usually looks a bit more scattered, at least in my experience). Perhaps he injured himself and picked up a secondary infection. :(

Injury-wise I didn't spot it before due to the top down view of the pond. The previous owner also used blue dye so it isn't easy to visually inspect. I'm beginning to notice that there are ornaments at the bottom though that I'll check for potential injury risks. I'm also going to slowly eliminate the blue dye so I can watch everyone more too.

Blackspot is another great point. We don't have any snails but being outside we can't rule out a parasite. Everything I've read though says it is pretty rare and isn't super deadly (unless I'm wrong). I guess I'll monitor the other fish closely and dose with Praziquantel if concerning symptoms emerge.... we're also heading into winter so hopefully any flukes will be shutting down / dying off.
Thank you, yeah he was moved to this pond from our old home.

I'm a bad estimator of size but he was significantly smaller than the others.
(Others were a good 10-15cm he was in the 5cm or less range probably). Two other fish about his size are getting along fine for now.

Thanks for sharing the nitrification cycle, I think we're good on it (water params look as expected). If anything, I guess there could of been a temporary ammonia spike although I've seen no evidence to suggest it.
Glad the rest are doing well. Maybe he got caught in something and was injured. Taking a good look around, and feeling for stuff that could hurt a fish. Honestly just having them dart into something because they are startled /scared can be an injury.
Bit of an update.

All fish behaving fine but the largest (an original from the pond) seems to have the same mark on his side.

Still no ammonia registering in the pond. I'm guessing a potential parasite now, I'm going to do a water change and run prazi on it.

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