About a week ago i decided to buy 5 fancy goldfish. I noticed that there were fish in the tank ,where they were being held, that i think had hith or some other disease. I got the fish home and had them for a couple of days. i woke up one day and noticed black spot on one of them. Shortly after that one could barely swim. About a day after that it couldn't swim at all but it was still breathing. The next morning i found it dead and another fish had the black spots so i quickly but it in a hospital tank. It's now dead. Now two more of the fish have the black spots.

If i take all the fish out of the tank and put new fish in will the water be contaminated? Do i have to take all the water out and restart the nitrate cycle?

Oh dear, you have done something that you should never ever do, purchase fish from a tank that Contains ill fish. I am surprised that the fish shop displayed fish that had obvious signs of disease, this is very unprofessional and I would certainly not buy from a fish store like that. Whereas blackspot isn't normally a deadly disease, you obviously don't want in your tank. I think you should take the remaining fish back to where you got them from and tell them exactly what has happened. If they are responsible, they will replace the fish, or give you a refund. if this is indeed blackspot, it isn't considered to be a serious condition and treatment isn't really necessary. it is actually caused by a small parasites that often occurs on newly imported fish. therefore, do a good water change, and just replace your fish from somewhere else, or a tank that doesn't contain ill fish.

however, having said all this, could you possibly post some photographs so I can do some comparisons to pictures I have here, even though I've said this is blackspot, I am only going by your description

I have no experience of blackspot. OFL if it is caused by a parasite why does it need no treatment. Does it need a part of its life cycle that cant be achieved in a tank. Also can it be transfered to any fish you may already have in a tank

As OFL says dont by fish from a tank with ill fish or dead fish. However fish stores regularly check their tanks for dead fish, so just cause there isnt any in there doesnt mean none have died that day so best go by the look of the fish and their behaviour. My golden rule is 'if in doubt dont buy'

another one died

here is the remaining sick one

s262.photobucket.com/albums/ii111/evila727/

ignore the clown

you can see the spots on its fins and in some of the pics you can see the spot on its gills

if i get the fish out of my tank is the water in my tank infected?

I guess it would depend on the lifecycle of blackspot. The worry is it may infect your other fish. That was why i asked OFL about why treatment isnt deemed necessary.

Unfortunately i cant see the pic. I keep getting the error message that photobucket is taking too long

photo bucket loaded for me and it said the site was down for maintenance

the disease isn't as developed on the fish i took pictures of

I'm less concerned about the fish and more concerned about the tank

i do not have any Oscars in this tank but i plan on buying some soon. I got the gold fish thinking the worse case scenario the Oscars would eat them.

fish are in another tank now
IS is OK to put Oscars in the tank?

I personally would leave the tank with the fish it has in it for a couple of weeks to make sure none of the other fish get the black spot

all the fish that were in the tank are dead. RIP fishy 1-5 (they didn't have names)

If the tank has not had any fish in it for more than 24hours then all the good bacteria in the filter will have died and you will need to cycle the tank again. If i was you i would give everything a really good clean and let it all dry out and then start again.

You have 2 choices, 1 is get some fish like danios and cycle with them and then get oscars. Or 2 add some ammonia remover like stress zyme or ammo lock and add oscars (i have no experience of this but others have- OFL used it for a small tank)