White mouth and not eating

Tank
What is the water volume of the tank? 50G
How long has the tank been running? A few years
Does it have a filter? yes
Does it have a heater? yes
What is the water temperature? 79 F
What is the entire stocking of this tank? (Please list all fish and inverts.)
Pleco (not sure of its exact species), 4 swordtails, 6 guppies

Maintenance
How often do you change the water? Once a week
How much of the water do you change? 10-20%
What do you use to treat your water? Prime
Do you vacuum the substrate or just the water? Both

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? API master kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia:0
Nitrite:0
Nitrate: 5-10
pH: 7.8

Feeding
How often do you feed your fish? twice a day
How much do you feed your fish? enough for the fishes to finish it within 2 min
What brand of food do you feed your fish? hikari and tetramin
Do you feed frozen? no
Do you feed freeze-dried foods? no

Illness & Symptoms
How long have you had this fish? a week
How long ago did you first notice these symptoms? 2 days ago
In a few words, can you explain the symptoms? Seeing it's mouth white and am not confident to say if it's always been there but my other swordtails don't have it. During feeding, it will go after food but spit it out after eating it. Also noticed it's not growing as plump as the other swordtails but I'm still observing them.
Have you started any treatment for the illness? no
Was your fish physically ill or injured upon purchase? no
How has its behavior and appearance changed, if at all? Swims normally and hangs out with other fishes but sometimes hides away in a corner by itself.

In the picture the one I'm concerned about is to the right.

 

Looks like the problem might be "Mouth Rot", a common bacteral infection.
You might up the weekly water change volume to 35% - 50%.
The Swordtail might survive the infection without dosing antibiotics, but will likely lose some mouth tissue.

 

Is it contagious to other fishes? I don't have a QT at the moment.

 

It's not readily contagious, but don't allow tankmates to nibble on the corpse of an infected fish.

 

Thank you! Is there like a specific brand of anitbiotics that's really good for this?

 

You might try Microbe Lift Artemiss. Petco sells it, but the Swordtail might heal up without meds.