Pond Goldfish 'Gasping' at Dawn and Dusk?

by Ruth

We have a raised pond and last year the three original goldfish had lots of babies, about 50 survived the winter.

At dawn and dusk they all come to the surface and appear to be gulping. I was panicked that they needed oxygen, the water is fine according to dip stick, there are aerating plants and solar aerator and filter (which obviously only work in the day).

The water was cloudy so it's been treated and is clearing but no change in the behaviour. During the day there's no issue, even in really hot weather we've had recently.

They're eating and none have died, most are changing from black to gold so appear to be growing and developing fine.

I'm just a bit stumped - are they eating?? I've noticed that tiny bits of algae (i think!) appear on the surface as the water cools into the evening and in the morning it's clear, so wondered if they were eating that? Or the little midges that land on the surface?

If they are gulping for air then I'm not sure how to help them as no way to get power to the pond for non-solar aerator.

I've rehomed 8 fish and potentially another 10 incase it's overstocked, but wouldn't they be gasping continuously rather than just at dawn and dusk?

All help gratefully received!

Grant's Reply

Hi Ruth

Goldfish gasping at the surface from lack of oxygen should be fairly obvious because of their rapid gill movement, little "swimming" with the fish staying relatively still, and all the fish behaving the same way.

Goldfish at the surface feeding wouldn't have rapid gill movement, and they would be seen pecking at food before moving to another location.

As for the dawn/dusk appearance of the fish, normally I would say that because the pond has aquatic plants, and aquatic plants absorb oxygen at night, a pond has low dissolved oxygen in the morning.

But this doesn't explain the behaviour at dusk when the dissolved oxygen level should be high due to the plants, aerator and filter.

If you feed the fish at dusk, the fish may just be feeding, unless the bottom of the pond has a large amount of decaying mulm on the bottom which depletes oxygen as the water warms during the day.

As a general observation, and not knowing the size of your pond, the bio-load has been increased substantially. 50 small fish would probably equate to 5-6 more adult fish, and the pond has to process that waste.

In regard to the solar filter and aerator; a filter must run 24/7 to build up beneficial bacteria that process waste (ammonia). If it isn't running at night, that process won't occur. All it is doing is circulating the water.

Having the aerator running at night would increase the dissolved oxygen level.

Could a storage battery system be set up that the solar power trickle charges during the day?

So, bearing all the above in mind, I would:

1. Observe the fish carefully, and decide whether it is feeding behaviour or gasping. And it could be both, gasping in the morning, feeding at dusk.
2. If it is a low oxygen level, test the water. You mentioned a dip stick test, but test "strips" are inaccurate; use a water test kit that has solutions and test tubes. Test for ammonia, which should be zero.
3. If ammonia is detected, a large water change is required immediately. Lower the fish population.
3. If ammonia isn't detected, lower the fish population anyway, and check how much decaying mulm is on the bottom of the pond.

You could test the pond water dissolved oxygen level using a test kit available from better aquarium shops. The highest level will be 10 ppm (parts per million). Anything less than 4 ppm, and you have a problem.

Let me know how you get on in the comments.

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