Post pictures and video of the fish?
If you use a mobile phone to film the fish, hold the phone horizontally so the footage fills the entire screen. Upload video to YouTube, then copy & paste the link here.
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How long has the tank been set up for?
What is the ammonia, nitrite, nitrate and pH of the water?
How long have you had the fish for?
How long has the fish been acting unwell?
What other fish are in the tank?
Have you added anything to the aquarium in the 2 weeks before this started?
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?
What sort of filter is on the tank?
How often and how do you clean the filter?
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BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.