Pond fish suffering from GBD?

[WaterPig]

The setup: I have two goldfish in a pond liner. In order to move it outside for the summer, I drained and refilled most of the water. It's a bare-bottom liner with accumulated mulm, leaves, and one piece of wood. They've been living in there for about 3 years. There are some surface plants and a few below (moss + java fern). The sides are also covered with algae. Water quality tests seem good: 0/0/0. The pond has good aeration, though the O2 level this morning was ~6 mg/l in a salifert test (unsure if I trust those). I'm using well water to fill the pond - I use the same water in my other tanks with smaller fish, no issues there.

The problem: Both fish appear to have gas bubble disease (GBD). They have formed bubbles in their fins and behind their eyes. It looks consistent with photos I see online. I suspect at this point neither will make it.

The question: How did I mess this up, and is there anything I can do?

Edited August 21 by WaterPig

[Zac]

On 8/21/2023 at 9:11 AM, WaterPig said:

The setup: I have two goldfish in a pond liner. In order to move it outside for the summer, I drained and refilled most of the water. It's a bare-bottom liner with accumulated mulm, leaves, and one piece of wood. They've been living in there for about 3 years. There are some surface plants and a few below (moss + java fern). The sides are also covered with algae. Water quality tests seem good: 0/0/0. The pond has good aeration, though the O2 level this morning was ~6 mg/l in a salifert test (unsure if I trust those). I'm using well water to fill the pond - I use the same water in my other tanks with smaller fish, no issues there.

The problem: Both fish appear to have gas bubble disease (GBD). They have formed bubbles in their fins and behind their eyes. It looks consistent with photos I see online. I suspect at this point neither will make it.

The question: How did I mess this up, and is there anything I can do?

If this is a new issue that would be odd. Did the pond undergo any changes before they got sick?

[WaterPig]

After doing a lot of searching I found this video, which may explain the problem. I really don't understand the chemistry here. If correct, the issue was doing a substantial water change from cold well water without properly aerating it first. I've started removing water from the pond, aggressively mixing it (pouring it from one bucket to another from several feet above), and then returning it to the pond. After many many cycles of this, the fish appear to be more active now... but that may be wishful thinking.