The Corydoras has fungus on its body. It is not Columnaris.
Columnaris is a bacterial infection that usually infects the mouth of fish. It starts off red and makes the lips look inflamed, then gets a whitish film over it and then fungus will often get involved too. The Columnaris bacteria will quickly eat away the tissue on the fish's mouth and spread to the jaw, throat, gills and sinuses. This occurs very quickly, within a few days and most fish are dead within 48 hours of showing signs of Columnaris.
Columnaris does not normally affect other parts of the body, and is relatively rare in clean established aquariums that have not had any new fish added.
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The fish has been damaged and a fungal infection has set up home in the wound. Salt is a good treatment for most fish but Cories don't like too much. If you can't get medication today then add 1 level tablespoon of rock salt or swimming pool salt for every 20litres of tank water. Then try to track down a medication with Methylene blue in, that should do the job but will stain the silicon blue.
Continue treatment for at least 1 week, preferably 2 weeks after the fungus is gone.
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 of the tank, measure from the top of the gravel/ substrate to the top of the water.
Remove any carbon from the filter when you use medication, and increase aeration/ surface turbulence to maximise oxygen levels in the water.
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Does the fish live under any sharp rocks or driftwood? If yes, then remove the sharp objects and dispose of them or smooth them off so they don't cause further damage.
How long has the tank been set up for? If it has finished cycling, you should look at getting more neons and Cories, but don't do it until this fish is healthy. Neons and Corydoras need to be kept in groups of 6 or more, neons do much better in groups of 10 or more. Same with the harlequins.
When you get any new fish, quarantine them in separate quarters for at least 2 weeks (preferably 4 weeks) before you add them to the main tank.
Increasing the general hardness to about 150ppm will help your guppy. A Rift Lake water conditioner (in powder form) can be used at 1/4 to 1/5 strength to increase general and carbonate hardness, and will cause the pH to go up a bit, but it shouldn't be a big issue for the fish you have.