Spots on my goldfish

Question
QUESTION: We have a 10 gallon freshwater aquarium for about 4 weeks now. There are 2 1-1.5" bottom feeding sharks and 2, 2-3" goldfish. One goldfish is completely gold and the other we bought with black markings on his fin and tail. Not a disease, just his markings.
That one is the bigger of the 2 goldfish and has black or brown spots all over now. Last week it was in a few areas but now it's over 1/2 of him. It's not limited to just scales either, some if his smaller fins are also changing color.
I read about Black Spot and the tank had gotten a little dirtier than normal. We were suppose to only have to change the filter once every 2-4 weeks and it had only been 2 weeks but we went ahead and changed it as well as adding another gallon of treated water (same temp as tank) due to evaporation. We time the feedings so I know I'm not overfeeding them and they are definitely not overcrowded with only 4 fish in a 10 gallon tank.
It was black spot, why is 'he' getting worse?
Plus, if it was ammonia, they showed no other symptoms as described with Black Spot and the other fish are doing fine. The other goldfish gills darkened slightly but they are already returning to normal. How could just one fish be burnt if it was ammonia?
I have no clue here and I'm new to fish. I owned them when I was much younger but these are for my 3 year old daughter. Any help would be greatly appreciated.
We now change the filter and add treated water every week but he's still getting more brown or black areas on him. He's swimming and eating great though so I'm clueless.
Thank you

ANSWER: 10 gallons will be too small when the goldies get bigger, but for now they should be ok...

never add warm water to cold water, it take out all the oxygen in the cold water and can kill your fish (i say this because you said you added water the same temp as the tank)

so to the problem...are the spots coming off of him like pimples?? also how big are they, and do you have a picture you could attatch?
if it wont let you attatch it my email is laurensq@hotmail.co.uk and you could send it that way?

that way i can diognose your fish better, and is anything out of ordinary with him, e.g not eating swimming werid ANYTHING??
and how long was your tank set up before you added the fish and how many fish did you add at once??

sorry to answer your questions with questions but if you treat a fish wrongly it can be fatal..

---------- FOLLOW-UP ----------

QUESTION: I an send a picture later but they are in my daughter's room at the moment and she's asleep. He is not acting weird or anything out of the ordinary. My tank was set up for recommended week before we added new fish. We added four at first but a little one died and we made a bad choice from the store with him. He didn't act good when we got him home. We replaced him in 2 days with this guy that has the problem. He didn't have the problem then or for two weeks after that. He eats great, swims all over the place, no redness, no swelling or anything else.
The spots are not raised but like his coloring. They aren't pimple like or coming off.

I read that when you change out the water, to make sure it was the same temperature as the tank water. We use 2 thermometers to check this. We have a tank heater that keeps the tank at 78degrees at all times (as suggested) and I ensure that the water we add is at 78degrees and treated before we add it.

Any idea why we are having to change the filter every week vs what it recommends? It's brand new and the one that came with the tank.  

Answer
ahhh i know what it is!!
goldfish are born black, then they usually go orange (i breed them)and the balck grows out so the eventually just the fins are black, then that grows out, anyway while this is going on they get there life time colouration, im think its just his colouration!!

the filter will be because goldfish produce ALOT of waste so need filters that are stronger than needed for the tank size.

does your fish kinda look a tad like this?? with the blackspots

http://www.exoticgoldfish.net/images/pic%20-%20breeds-shubunkin1.jpg