Fish naturally have a thin clear layer of mucous over their body and fins. It helps them slip through the water easier and provides a first line of defence against things in the water. When they are stressed out by something in the water, they increase the amount of mucous and it can look like there is a creamy white film over the body and fins. Like your fish in the picture.
If fish are ever stressed out and or showing a creamy white film over their head, body and or fins, test the water, and do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
The big daily water changes will dilute any harmful substances or disease organisms in the water and help the fish.
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There is no point treating fish with chemicals unless you know what the problem is. Most chemicals are poisonous to fish and other organisms, including people. If a fish has ammonia poisoning and you add a chemical medication to treat bacteria, fungus and protozoans, you make the fish worse.
Chemicals and medications should only be used if the fish is actually sick, and you need to use the correct treatment for the ailment.
The following link has information about what to do if your fish get sick. It's long and boring but worth a read when you have some spare time.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
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