Emperor tetra's mouth is red, and color is fading a bit

not sure what's wrong with this emperor tetra, it acts normally, but the mouth is red and its color is a bit faded.
20 gallon tank
6 emperor tetras
1 dwarf gourami
Ph 7.0
Nitrates 0
Nitrites 0
Ammonia 0

EDIT: Found a small red line between the bottom fin and its stomach, it also looks a bit bloated.

It has a bacterial infection (possibly Columnaris).

How long have you had the fish for?
Have you added anything to the tank in the last 2 weeks?

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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 filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.

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.

Add 2 heaped tablespoons of rock salt for every 20 litres (5 gallons) of tank water. When you do the daily water changes, add salt to the new water before adding it to the tank so the salinity (salt level) remains stable.

Monitor the fish over the next 48 hours and if it gets worse, you will probably need anti-biotics.

I've had the fish for about a month, noticed this in the past 2-3 days, i've only added 2 new plants to the tank.