Red sore on my Mickey Mouse Platy

[MoonyTonight]

Hello all. First things first, here is the filled out question form:

Second my Female platy who I thought might be pregnant has developed a red sore on her side today. No other fish is affected and shes not acting TOO sick. Just a little shy.


Housing
What size is your tank? 50 US Gallons
What temperature is your tank? 76 F
Does your tank have a filter? Canister filter Marineland HOT Magnum rated for 55 gallons.
Does your tank have an air stone or other type of aeration? I aim the output of the filter at the surface and that gives the tank a bit of aeration.
Is your tank heated? Yes

Food
What type of food do you feed your fish? Frozen bloodworms and Brine shrimp San Francisco Bay brand, and Omega One Tropical Flakes as a staple. Sometimes a live gnat or two to whoever grabs it. (They are in the vial you can buy at petstore.)
How often do you feed your fish? Pinch of flakes twice a day. If not using flakes then frozen food once a day. One day of fasting a week.

Maintenance
How often do you perform a water change? 10% weekly. 20% once a month.
What percentage of the water do you change when you perform a water change? See above
What type of additives do you add to the water when you perform a water change? Seachem Prime conditioner.

Water Parameters:
Ammonia:0.25 ppm
Nitrite: 0 ppm
Nitrate: 20 ppm
Tank had been cycled for about 4 months now. I have 9 platys 9 Black skirt tetras 4 Neon tetras 10 zebra danios and just a few days ago added 3 oto cats. The cats I think are what has bumped my ammonia up. Seeing as how my readings have been consistently 0 0 20 for months. I'm about to do a water change to tackle that ammonia, but I'm sure it didn't cause that sore.

Symptoms and Treatment
How has your fish's appearance changed? There is a red sore on the right side of my female platy right behind the pectoral fin.
How has your fish's behavior changed? Has become slightly reclusive. Still eating, but took longer to notice the flakes compared to other fish in the tank. Chased the food though.
When did you start noticing the symptoms? When I came home from work 30 minutes ago.
Have you started treating your fish? If so, how? Not yet
Does your fish have any history of being ill? No
How old is your fish (approximately)? Had her for 3 months now


I'm including a link to a YouTube video i got of her. Please give me any insight you might have and I'll be monitoring this thread. I can provide more video or pics just let me know. Ask me anything.

http://www.youtube.com/watch?v=9XlzhJhOJus Can be hard to see but its a red sore. Please please please watch in HD

[MoonyTonight]

Too late to edit original post, but I've done a small water change and used some of my stress coat + on the tank. Any help is appreciated guys... seriously.

[MoonyTonight]

Okay, I've got home from work again this morning and she is still alive. Everyone's doing alright and the sore hasn't gotten worse, but it hasn't gotten any better either.

I don't expect answers in a second, but i figure I would have at least one person trying to help me by now.

So if a mod could just close this thread or do whatever with it. I'm going to try to ask somewhere else. I need to know any possible course of action before its too late to do anything, and playing the waiting game isn't fair for her.