Can anyone identify why this fish is unhealthy? And advice what to do next?

[Subete]

The above link (mod edit: broken link removed)
takes you to a youtube video of my fish behaving oddly.
Could you please help identify and give advice as to how to cure it?

I will add that just before this happened all of my fish suffered from white spots, but after using medicine it seems that all other fish are ok.
This one (and 3 other of the same species) now again seem unhealthy.

All help will be appreciated!

Tank
What is the water volume of the tank? About 70L
How long has the tank been running? 4 weeks
Does it have a filter? Yes
Does it have a heater? Yes
What is the water temperature? 25C
What is the entire stocking of this tank? (Please list all fish and inverts.) Don't know all the names but some are... Gubby, Catfish, Shrimp

Maintenance
How often do you change the water? Only started 4 weeks ago, but I change about 20l every 2 weeks
How much of the water do you change? As above
What do you use to treat your water? Tap Water treatment product for fish tanks
Do you vacuum the substrate or just the water? Don't even know what that is... again, just started

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? I didn't
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: No idea
Nitrite: No idea
Nitrate: No idea
pH: No idea

Feeding
How often do you feed your fish? Twice a day small portions
How much do you feed your fish? As above... don't know how much is it in grams
What brand of food do you feed your fish?
Do you feed frozen or freeze-dried foods? None of this

Illness & Symptoms
How long have you had this fish? 4 weeks since the start
How long ago did you first notice these symptoms? A few days ago after the White Spots treatment
In a few words, can you explain the symptoms? In a video you can see the fish 'shaking', and only stays at the bottom of the tank unless approached. After the previous illness treatment all other fish seem to be ok now, but this species is acting worse.
Have you started any treatment for the illness? Don't know what the illness is, so no
Was your fish physically ill or injured upon purchase? No I don't think so
How has its behavior and appearance changed, if at all? Colours became paller and it seems struggling to swim, plus shaking

 

[SarahPar]

Could be stress or neurological. Best to do frequent water changes incase it is from stress.

 

[Idkitsathrowaway]

You will NEED the water parameters! I suggest the API master fresh water test kit. Its really good. Vacuuming is when you take a hose connected to a wider clear plastic tube that you put in the gravel to pull the poop out. 2 we can try to identify the other fish send photos.

About your fish: It looks like the fish it almost having a seizure(may be wrong). and it also looks like your fish is trying to float for a min and swims back down. It may be a mild swim bladder infection. White spot is ick. It is a bacteria that is always in water. It only shows up when your fish has a weakened immune system, or a imunial glitch. This can be caused buy prior infections(like swim bladder) or due to stress.

 

[Flyfisha]

Many shops will test water for free.
With the tank just being 4 weeks old and you writing of changing 20 litres every 2 weeks it’s extremely likely the tank has toxic ammonia in it.
Remove a little cup of water for testing , but remove 30 litres of water as soon as possible and replace with temperature matched conditioned water.

If you did not add an ammonia source to the tank for around 8 weeks before you added fish the tank does not have the colonies of bacteria needed.

 

[DoubleDutch]

Is this the only cherry barb ?