Life or death for my baby pet goldfish?

SORRY IF THIS IS SUPER LONG BUT I JUST WANTED TO BE SUPER SPECIFIC AND DETAILED ON MY SITUATION.

I DON’T WANT MY BABY FISH TO DIE

Okay so I will explain what has happened so far, and what I am concerned about now.

So I have had this goldfish for over 1 week. Which I have named Kimpy
I got Kimpy 10/11/2013.
So when I got Kimpy I didn’t quiet have the tank set up for him and I left him in the baggy overnight (opened) floating in a 10 gallon gallon tank I have.
Then Saturday morning I let him out and placed Kimpy in the 10 gallon with my other fish. (I have a fan tailed goldfish and 3 koi fish.) (Yes, they were sharing a 10 gallon tank, I know that it’s wrong and unhealthy for them to share such small tank. But I’ve corrected my mistake and moved them to a 30 gallon tank. I know that wont last long especially for the koi but that’s all I can do for now. Besides I plan on making a small pond for them next Spring/Summer.)

…Okay going back to the main subject…

I spent most of my time outside preparing the tank for my new fish. When I went to check up on Kimpy I noticed he was trying to hide from my other fish and he had tears in his tail fin. I rushed on putting the tank together and placed him in the tank (with water conditioner and everything else a goldfish needs). So I placed Kimpy and my fan tailed goldfish who’s name is Rusty. So I put the two of them in the 30 gallon tank.(Yes, I left the kois in the 10 gallon tank.) I thought if I put all the fish together again they would bully Kimpy. So I left the two goldfish over night together. The next morning (10/13) I got up to feed all my fish and when I went up to the tank which had the two goldfish I noticed that Kimpy had more tears in his tail fin. I quickly got Rusty out of that tank and put him back in the 10 gallon.

So the whole day (10/13) Kimpy was alone in the 30 gallon tank but he didn’t moved very much and he didn’t eat either. I was worried about him but I didn’t really think much of his tail fin thinking it would heal on his own.
(Because this isn’t the first time I’ve dealt with fish bullying. My kois’ use to bully Rusty. My kois’ started ripping Rusty’s tail fin about a month ago, and because of that I started restoring an old fish tank which is the 30 gallon I keep them in now. Because my original plan was to separate my kois’ from Rusty. But now they don’t bully him anymore, and Rusty’s tail fin has healed now. I messed up on my original plan by getting a new fish. X.X)

So me thinking Kimpy’s tail fin would heal on its own I let him be. Another day goes by and it’s Monday (10/14) he’s still not moving much and he still has no appetite. His tail fin still looks bad, and it had a white murky line on it. So, I was really worried about him and did some research online. While, I was doing my research I found out that Kimpy has fin rot. But by the time I found out Kimpy had fin rot it was too late, because all the pet store were closed and I couldn’t go and get the right medicine for him.

I read online that fin rot is due to poor water, bacteria, and many other factors. Fearing that it may be the water I took him out cleaned the tank and placed my other four fish in the 30 gallon.
I also cleaned the 10 gallon and placed Kimpy in that tank. I was surprised that when I placed Kimpy in the 10 gallon he was a lot more active and he started to eat more. The very next morning (10/15) I went to the store and bought API’s Melafix, which treats bacterial infections, fin rot, and many other fish diseases.
I’ve been fallowing the directions on the label and Kimpy was getting better. He swims around, eats the little food I give him, and he also grazes on the live plants I have in the tank.

What I am concerned about now is that one of the signs that your fish has fin rot is that there is a white murky line on your fish’s tail and/or fin. The white lines have been fading slowly with the medicine I’ve been giving him, but now he has a black line on his tail fin. Which he didn’t have before.

I’m worried that it may be a fungal infection but I can’t find anything similar to what he has. All the pictures I find of fungal infections on fish have to deal with white spores or cotton ball like patches on the fish.

I do notice the left over food that Kimpy doesn’t eat does look like it may start to grow some kind of fungus.(http://www.hubbardbrook.org/mirrorlake_kids_tour/images/fungifishcut.png I know this is a dead fish with fungus on it. I picked this picture because at the very end of the fungus how it has that very thin transparent “hairs” on it. That’s what the food looks like.) I’ve been removing the uneaten food that looks similar to the picture.

It can’t be that the pH level is too high or too low because I test it everyday and it has been the same. The pH level I have is perfect for goldfish. Same goes for the ammonia level. I test it everyday and it is always at 0 ppm. The only thing I cannot test for is the nitrite level in the tank. More than likely it is also low.

I still don’t understand why he has this black line on his tail?

Can someone please help me and suggest possibly a different kind of medicine to aid with Kimpy’s health?

And whether or not I should remove the live plants in the tank?

And if any what kind of disease Kimpy might have and how to treat it?

I also want to add real quick that my other four fish are doing just find with no tears or any other disease.

I would also like to thank you for taking your time to read this very long essay.

If you would like pictures to see how Kimpy’s tail looks like all you, I will post some pictures, all you need to do is ask. :)