Yesterday, I noticed a black/blue spot on the tip on my fantail's tail.  It seems a little slower than usual, as does my other fantail, but they are both eating just fine.  The spot seems to be a bit bigger today.  I really need help figuring out what it is. 

My tank is 15gal, set up for about two weeks.
I have one other fantail, two cory cats, and one mystery snail.
I just tested the water and below are the results:

Ammonia: >.02ppm
NitrIte: 0ppm
NitrAte: about 30ppm
pH: about 7.4
Temp: 75F

I do a about a 25% water change weekly and use kordon NovAqua Plus water conditioner.
I just installed (last night) a new filter.  It's an Aqua Clear power filter for 5-20 gals. It does 100gpm.

Picture 1 shows the spot.
Picture 2 shows my indicators.
Picture 3 shows both fantails so you can see they look healthy otherwise.

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Short backstory: I worked for a small office (3 people) who were struggling with sales.  My manager decided one day to Feng Shui the place which apparently meant getting a goldfish.  She bought him, a net, water conditioner and a one gallon bowl.  I noticed the poor fish was always gasping for air so I began daily water changes. I soon bought him a 5.5 gal tank with filter; so my manager quickly bought another fish.  The company went out of business and since I was the only one who actually cared about the fish, he was given to me.  I am really trying hard to do things right and make a great home for them.
My dad had freshwater when I was little then graduated to salt water. Thankfully, he is very knowledgeable, but we live several states away so it's hard for him to help. 

That spot is nothing serious, anything black on a goldfish means bruise, it's healing. Your water quality seems decent, but you do have the nitrates a tad high, I say perform a 50% water change and vacuum the gravel

Also, you'll need a bigger tank for all these fish I would suggest a 25 o 30 gallon if you can find one, but for now your fish will be fine as long as you test the water and change out 50% every time the nitrates reach 30

Great! In case you bump into any other trouble, feel free to ask any questions you might have