Mystery goldfish die-off in established 600 gallon pond

FishyJoe

Background: A year ago I bought my house, which has a roughly 650 gallon pond built into the back patio. It was stocked with a bunch (30ish) of small goldfish (1"-2.5") and one Koi (8"). They were all healthy and until recently (a year later) all stayed alive. (I don't need the over-stock lecture, the pond was this stocked when I moved in, and I fully plan on culling the load as they grow).

Specs: pond is 6'x6'x30" with a 12" ledge on one side. No substrate. Box filter with sponges and bio-balls. The pump is only about 500GPH. I have it running a UV sterilizer (pond gets mostly-sun, and was pea soup when I moved in) and a spitter. In addition to the fish, it has three small lily-pad plants in baskets, and a corkscrew rush in a pot. It also has two bullfrogs which have survived the winter and come back. I regularly dredge out dead leaves, and do monthly water changes (25-50%).

The problem: About three weeks ago, the girlfriend took the niece and nephew to the state fair. They won goldfish, which their mom wouldn't let them keep. Feeling bad, and the two goldfish looking outwardly healthy, I floated the bags, acclimated them, and netted them into the pond. About 5 days later, I found one of the two state-fair fish floating in the morning. From then on, I lost 1-2 per night. They all look fine: fed but not bloated, no physical trauma, no rot or fungus, no visible parasites.

What I've tried already: Since they all seemed to die overnight, my intiial thought was low-oxygen overnight (it was kinda hot, but not any hotter than it had been all summer). I changed the discharge from the UV sterilizer to make a ton of surface disruption. The deaths continued. I next increased the frequency in which I cleaned the filter box to every-other day, and did a 80% water change.

That was a week ago. Aside from a minor algae bloom, things seemed fine, but when I got home from work today I had a goldfish floating at the surface: alive but only occasionally gasping or kicking his fins. He's definitely on his way out. To this point, I hadn't actually tested the water, figuring it was an established pond and I had just done a massive water change. So I tested the water expecting to somehow find toxic levels of ammonia, but what I found was no ammonia, no nitrite, and less than 5 ppm Nitrate.

So basically, I came here because I'm out of ideas. I am considering dosing for parasites, but I haven't actually seen any external signs of them, nor any bloat suggesting internal ones, but it seems like Occam's razor since all this started with the addition of two new (questionably sourced) fish. That said, I don't want to subject my fish, my plants, my bullfrogs, and the rest of the little ecosystem to Rid Ich or copper on a guess if there are other things I haven't thought of.
 

Junne

Honestly, the fish that they get from the fair are usually feeder fish and probably not in the best health as it is. I doubt its anything you did.

Sounds like a lovely pond you have there

Welcome to FL
 

FishyJoe

Thanks for the welcome!

I should have been more clear. The first fish to die was one of the two state-fair fish, but the subsequent 8-10 have been my established, bigger, previously healthy comets.
 

Junne

Thanks for the welcome!

I should have been more clear. The first fish to die was one of the two state-fair fish, but the subsequent 8-10 have been my established, bigger, previously healthy comets.

Well it might be possible that the fair fish gave something to the others then. Especially because they are feeder fish. Did you QT the fair fish first before putting in your pond? I think I read that you just put them in there after acclimating, right?
If that's the only thing you added, then its probably a good chance that's what happened. In any case, that sucks about your comets
 

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