Why did all 45 goldfish (and 2 mosquitos) in my pond suddenly begin an overnight, 4-day die-off?

Question

I have a pond, which contains the water volume of 6000 liters (1600 gallons). I am very frustrated: my 45 or so goldfish and several mosquito fish were so happy. Then, literally overnight, they stopped eating and within days died off.

The day the trouble started, the outside temperature soared to 35 °C (95 °F), but the water seemed cool enough. Moreover, the center of the pond is 120 cm (4 feet) deep.

In happier days whenever I’d feed them they would snap and race around their pond. The pond is an in-ground hot tub covered with a non-toxic triple liner designed for fish that cost me $300. We live in Pasadena, CA in USA where the weather is generally moderate.

The goldfish were rescue fish destined to be eaten by Pet Smart customers’ carnivores. So they were at least spared whatever terror was in store for them.

After they started to die off, I changed almost all of the water but not all because there were some fry I was attempting to save. In hindsight I wish I had gone 100%.

I repeatedly tested the water with a kit and also at PetSmart. Then, I put some new goldfish in—all of which died within days. The water was hard, so I added the proper amount of salts. The pH was around 7. The nitrite level was borderline stressful, so I added a bottle of stress coat. Still, no luck. All other readings (ammonia, etc. were normal).

I had made a filter using bio balls and ran a small pump constantly with ok aeration. Admittedly it could have been better, as the balls never really looked that dirty. Moreover, because a raccoon had punctured the liner, only 3 months earlier I had completely drained the pond and did with no lose of fish.

After the disaster struck and following the water exchange, I built a massive filter and added a heavy duty pump that made the water crystal-clear—but it was too late. Another batch of new fish died.

After draining the pond, I realized too late that some of the fish that died had sunk to the bottom and possibly continued to contaminate.

Half the pond’s surface was covered with 2 types of water lettuce. I have now 100% drained the pond and have discarded the lettuce for fear that it is contaminated. Could the lettuce have caused the problem?

Raccoons were always trying to get the fish but didn’t seemed to have any success. Could they have introduced a disease?

Maybe there was a fish-borne disease? But no new fish had been introduced for months.

Answer 1

We likely don't have enough information.

However, there should be NO nitrite or ammonia in an established pond. The presence of nitrite means the presence of ammonia. Ammonia will come from the fish and decaying organic matter (e.g. dead fish, rotting leaves... anything that can decompose basically). Gross overfeeding could cause that, but it sounds like you've had the fish for at least months. Overfeeding would have caused a problem a lot sooner than that.

It sounds like until recently that you had no means of circulation and/or oxygenation. Warm water cannot hold as much oxygen as cold water. Without circulation and agitation, it wouldn't take much of a film on the surface of the pond to greatly inhibit gas exchange. So, that could lead to oxygen deprivation. That would not, in itself, explain your nitrites. So my best guess is: the water got so hot, some fish expired from oxygen deprivation. Those fish rotted which drove up ammonia and nitrite and further exacerbated the lack of oxygen, and you had a chain reaction fish kill. It's unfortunate but true that some fish dying can greatly worsen the situation for the remaining fish, and things can get out of hand quickly.

Then again, could be chemicals/poison too.... just no way to know probably. Get a good filter system that will aerate the water as well as convert ammonia and nitrite quickly, and try again. Start with just a handful of fish and only add more only after testing negative for ammonia and nitrite. (it could take a while).