My harlequin rasbora is covered in a white substance

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Country joe

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As above, and is swimming erratically
IMG_20240610_114059.jpg
 
Need a clear picture of the fish that is in focus.

Need a video of the fish showing its erratic behaviour. You can upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on each end.

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How long has the tank been set up?
How long have you had the fish?
How long has the rasbora looked odd?
Have you added anything to the tank in the 2 weeks before this started?

What is the ammonia, nitrite, nitrate and pH of the aquarium water (in numbers)?
What sort of filter is on the tank?
How often and how do you clean the filter?

How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
 
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Need a clear picture of the fish that is in focus.

Need a video of the video showing its erratic behaviour. You can upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on each end.

--------------

How long has the tank been set up?
How long have you had the fish?
How long has the rasbora looked odd?
Have you added anything to the tank in the 2 weeks before this started?

What is the ammonia, nitrite, nitrate and pH of the aquarium water (in numbers)?
What sort of filter is on the tank?
How often and how do you clean the filter?

How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
Tank is a 125 litre juwel rio with juwel internal, filter, sponges and bio, I check the sponges with every water change, I lightly washed two yesterday in tank water, they were not very dirty.
I have been bothered with hair algae so my tank has been blacked out for 6 days to try and get rid, yesterday lights were back on and everything was normal. I check fish this morning at 11am when lights came on, its when I noticed fish, it seems to have a small piece of white on end of nose as well as body, its now swimming normal, but not mixing with rest of fish. I do a 50% water change weekly, which I did yesterday, I gravel clean every water change, and I declorinate my new water. Tested water today at 1pm ammonia, nil, nitrite, nil, nitrate, nil, and ph 7.5.
Yesterday I added 3 mls api tap water conditioner, 4 mls bio boost, 20 mls serera flortina plant fertiliser, and 2mls ever ready liquid c/o2 all other fish seem okay, but I don't want this to spread.all dosing was below tank capacity except for api water tap conditioner.
 
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Yesterday I added 3 mls api tap water conditioner
This is not an answer to the white harlequin, but why did you add that much API Tap Water Conditoner? You said you changes 50%, and 50% of 125 litres = 62.5 litres or 16.5 US gallons. That amount needs 16.5 drops for chlorine which is less than 1 ml. Even for chloramine it would be less than 3 ml.
 
The white stuff on the fish is excess mucous produced by the fish because something in the water is stressing it out. It can be caused by poor water quality (ammonia, nitrite, nitrate or pH) but your water looks great. It can be caused by external protozoan parasites like Costia, Chilodonella or Trichodina but these normally cause cream, white or grey patches on the fish, and the white stuff doesn't cover the entire head, body and fins. There is something in the water upsetting that fish and if you did a water change yesterday and you noticed it today, then it probably has to do with the water change or something you added then.

Some fish are more susceptible to things in the water compared to other fish. That particular fish might have swum into one of the chemicals you put in the tank and got an overdose, which has upset it.

I would just monitor it over the next few days and see how it goes. It's not a disease and I would normally recommend a water change if you hadn't just done one. Just see how it goes. Post more pictures if it gets worse.
 
This is not an answer to the white harlequin, but why did you add that much API Tap Water Conditoner? You said you changes 50%, and 50% of 125 litres = 62.5 litres or 16.5 US gallons. That amount needs 16.5 drops for chlorine which is less than 1 ml. Even for chloramine it would be less than 3 ml.
Ouch I made a right mess of that, will know next time
 

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