Problem with Harlequin???

I noticed this morning that one of my harlequins has a white spot that sticks out from his body a little bit as seen in the pictures. I'm wondering if it is fungus, mucus or nothing? Thanks

Looks like possibly a loose scale due to some sort of injury to me. (from crashing into decor or maybe bullying)
What else is in the aquarium?

Upping the water change schedule is a good preventative against injuries becoming infected.

How big is their tank, what are the parameters, are there any other fish in there with the harleys, and what kind of decorations do you have? It does look like a loose scale but it's a little big and I can't be sure.

How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

How long have you had the fish for?
Have you added any new fish, plants or shrimp in the last 2 weeks?

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The fish has clamped fins, a white appearance to the top half of its body, and a small red patch & dots on the side of its body. This is either poor water quality or more likely a protozoan infection.

Check the water quality for ammonia, nitrite, nitrate & pH.
Do a 75% water change and gravel clean the substrate.
If the tank is more than 6 weeks old, clean the filter.
Wipe the inside of the glass.

If the water quality is good then it is a protozoan infection and any broad spectrum aquarium fish medication should clear it up. Any white spot medication should also do the job.

If you can't get to the shop for a few days then do a 75% water change and gravel clean the substrate each day until you get treatment. Alternatively, you can increase the temperature to 30C and keep it there for 2 weeks and it should kill the protozoans too. If you do increase the temperature, make sure you clean the tank and filter first, and increase aeration/ surface turbulence to maximise oxygen levels in the water.

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If you decide to medicate, then use the following to work out the volume of water in the tank:

measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.

There is a calculator/ converter in the "How To Tips" at the top 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 absorb the medication and stop it working.

Increase aeration/ surface turbulence when using medications because most of them reduce the oxygen levels in the water.