Disease/sickness or just inappropriate tank conditions? Advice needed please

I am trying to help a friend out with their aquarium. They have had a difficult experience with fish keeping so far, and they need help with their zebra danios. I have never kept danios so I can only offer what I am able to research online. I'm really hoping that those of you with experience with these fish or similar will be able to help us/them out. The petshops that sold her the fish gave her some bad advice, even that danios are ammonia resistant...

Tank: 35x60x30 cm (56L tank).
Water: 415 CaC03 mg/l (General hardness 415ppm or 23.2 dH).
Fish: 8 Brachydanio rerio (3 potentially sick)

I know there are issues with the tank size and the hardness is higher than ideal but she said that she'll be able to buy a bigger tank for them before the summer. In the meantime I've advised her to get some plants for the tank. I went back through our messages and I've listed all of the problems that she can see with the fish, as well as two short clips of the fish she took for me today.
There is one fish that is the worst, and two that seem to be getting sick.

• Jerking/swimming weird
• Resting at the surface of the water
• A little swollen
• Belly looks darker inside
• When its at the surface, it curls
• Redness
• Red anal vent

Video 1 (1 minute, 5 seconds)
Video 2 (10 seconds)


Tank history:

• Started with 4 goldfish
• 2 died in August/September
• Added a black moor goldfish and zebra danios in November
• 2 weeks later the 2 remaining goldfish died
• 1 week later the black moor died

Notes:
She has been performing regular water changes and there is no ammonia, nitrite or excessive nitrate.
She was sold the danios as cold water tankmates for the goldfish, and has added a heater recently to raise the temperature from 16 to 20 degrees Celsius.

I would do extra water changes of 50%+ and treat with de-wormer.

Is there any info about the fish that died?

Or


I have used both and they work (if that's the issue) and by the looks of them it's a good bet

Naughts said:

Is there any info about the fish that died?

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I can certainly ask her. She mentioned before that she thought it was swim bladder, I didn't know her then so I don't know myself.

Is there anything you can think of to ask her? I'll ask for anything she can think of anyways

Is there a filter in the tank?

What is the ammonia, nitrite, nitrate and pH in numbers?
What sort of filter is on/ in the aquarium?
How often and how do they clean the filter?

How often do they do water changes and how much do they change?
Do they gravel clean the substrate when they do a water change?
Do they dechlorinate the new water before adding it to the aquarium?

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The fish swimming with its head down and tail up could either be a swim bladder issue or air in the intestine. When fish take food from the surface (especially dry food), they take in air and that travels through the intestine and is eventually farted out. While the air is in the intestine, the fish can float up to the surface and has to spend all its time and energy swimming down.

Stop feeding the fish dry food for 1 week and feed it frozen or live foods instead. If the problem corrects itself (should only take 1 or 2 days to notice a major change) then it's air in the intestine and the owner will have to feed less dry and more frozen and live foods.

If the problem continues after a week without dry food, then the fish has a swim bladder problem and there is no cure for that. If that is the case, the fish should be euthanised.

Naughts said:

Is there any info about the fish that died?

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"All started struggling to swim, hiding out at the bottom and not eating.."

Colin_T said:

What is the ammonia, nitrite, nitrate and pH in numbers?
What sort of filter is on/ in the aquarium?

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"I'm not sure about the parameters in numbers other than what's on the master test photo. Hob filter.. I have a sponge filter to add too once I do the makeover. "

Colin_T said:

How often and how do they clean the filter?

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"Filter I do maybe once a month swishing the inserts in tank water I have taken out"

Colin_T said:

How often do they do water changes and how much do they change?

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"Usually do 40 percent ish once a week but I'll admit I'm not always exactly on time with it.. "

Colin_T said:

Do they gravel clean the substrate when they do a water change?

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"gravel clean with every second water change with a vacuum.. "
"once or twice I did take out the gravel and clean it as it was disgusting.. "

Colin_T said:

Do they dechlorinate the new water before adding it to the aquarium?

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"Yes I add a dechlorinator.. I have bought a new one now.. prime.. just I case its that.."

So I was messaging her again today, and from what she says the fish seem to have improved today.
"All healthy this morning... I wonder was it stress from the water getting cold"

I asked this evening if they were still appearing to be healthy and she said:
"All still behaving normally and redness seems to be gone"

The water tests are fine. The pH is high. There is 0 ammonia and 0 nitrite. There is a small amount of nitrate (5ppm) but that is nothing to worry about and wouldn't cause this. Nitrates should be kept as close to 0ppm as possible and under 20ppm at all times, so 5ppm is great.

re: cleaning the substrate with a vacuum, do they use a gravel cleaner or just an aquarium vacuum to suck stuff off the top of the gravel?

You want to use a gravel cleaner every time you do a water change so you keep the gunk out of the gravel. If you use a gravel cleaner regularly, you should never have to remove the gravel and clean it.

A slight drop in temperature should not cause this issue.

Monitor the fish and see how they go. If it happens again post more info and drop dry food for a week.