Black goldfish with its fin clamped down

Question

We have two small fish in our tank, they seemed very happy until about a week ago. They both became fairly inactive and had their dorsal fins clamped down against their backs. We thought it might be the filter in need of a change, so we replaced the cartridge and things started to look up for the gold one, but the black one is still looking quite bad. He spends a lot of time either at the very top or the very bottom of the tank.
Some Info:

They're in a 25 litre tank
They're both about 5-6cm long
There are two plants in there and one piece of wood for hiding in
A partial water change is done every week (changing 20%-30%). When the water is changed, the glass and wood is cleaned too
They last had a water change exactly a week ago
We've had them for about 2 months
The water has been tested at the local pet shop (Pets at Home) and everything was good
There is no water heater in the tank
After worrying about the filter (it was just a cheap one that came with the tank) it has been replaced with a new one. I read online that this can result in losing the "good bacteria", so I left the old one in there too (switched off)
They get fed every other day
The pet shop recommended adding some aquarium salt so I added one tablespoon (as recommended on the packet) a few days ago. This seemed to help for a short while

Any ideas what we could do to help?

rlb.usa · Answer

If you got the fish fairly recently, the most likely answer is that they were in poor health already when you got them from the store. This is a very common issue and not all pet stores treat their merchandise the best. There can also be pre-existing issues from where they were bred/raised before being transported to the store, and the transportation itself can be an issue, too.

Hanging out at the bottom, top, the general lethargy you mention, and fins not being displayed are all the fish's way of saying he doesn't feel very well. With the fins clamped, I almost wonder if there was an issue with Itch (did he seem to scratch himself at all)?

They're pretty goldfish, but everywhere I've seen fish, the goldfish always have the worst conditions as many fish are sold as "feeder fish" for other creatures. Your high-res photo was great but didn't reveal any clues.

Black goldfish with its fin clamped down

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