Hi and welcome to the forum
Can you post more pictures of the fish. Show the entire fish from both sides.
What is the ammonia, nitrite, nitrate and pH of the tank water?
How long has the tank been set up for?
What sort of fish is it? It looks a bit like a Betta?
How long have you had the fish?
What other fish are in the tank?
How long has the fish been like that?
Have you added anything to the tank in the 2 weeks before this started?
Was the other fish that died the same type of fish and did you get it from the same place?
Tell us about that fish and its history, including anything you tried to treat it with.
Did you clean the tank after that fish died?
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It looks like a bad bacterial infection on a Betta splendens (Siamese fighting fish). It will probably need a broad spectrum medication that treats fungus and bacteria. However, without knowing more details that is all I can suggest.
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Before you treat the tank, do the following things.
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
If you have big rocks or driftwood in the tank, remove these before measuring the height of the water level so you get a more accurate water volume.
When you measure the height, measure from the top of the substrate to the top of the water level.
You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" at the bottom of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will adsorb the medication and stop it working.
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. The water change 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 when using salt or medications because they reduce the dissolved oxygen in the water.