
The whitespot hasnt 'come' from anywhere, its a parasite that all fish carry. Like mange in dogs, it shows itself when fish are stressed.
An uncycled tank, which this is, will stress teh fish (as the ammonia and nitrites will burn the fishes gills).
A tank should take around 4 -6 weeks to cycle, sometimes longer. Not a WEEK!
What has to happen is the ammonia builds up in the tank. The filter develops 'friendly bacteria' which turn the ammonia into nitrites and then nitrites into nitrates.
Ammonia and nitrites are very poisonous to fish, you should have next to NO nitrites and absolutely NO ammonia in the tank before you put fish in.
NitrAtes are not nearly so poisonous, they still are though, but we remove those by doing regular partial water changes (partial mind you, not the entire tank contents!)
Occasionally you will need to change the sponges and floss from the filter, or clean it. To clean it you rinse it in OLD tank water, never ever tap water (chemicals in this will kill the bacteria) to remove large lumps of crud. To change it you take out HALF the old stuff and replace. Taking it all out will cause the tank to cycle again, probably damaging or killing your fish in the process.
Adding too many fish will also cause ammonia levels and thus nitrite and nitrAte levels to shoot up. BIG fish like plecs (and a great many are far too big to go in the average tank) create HUGE amounts of waste and can really naf up a tank if you arent aware of it. It is something of a myth that these guys will clean up the tank, they NEED feeding and they will produce loads of poo.
I would personally treat the remaining fish for the whitespot - turning up the temperature a couple of degrees will speed up the parasites life cycle meaning they hatch faster and meet the chemical treatment in the water quicker.
When they are clear for a week or more and the water is free (due to partial water changes) from the treatment, start monitoring the water quality with test kits - i use TetraTest 5 in 1 strips and a seperate ammonia tester - you may well see the tank cycle with the fish in it, you are looking for the ammonia levels to spike and then drop down to nothing.
WHEN the tank ammonia levels reach nothing, and the nitrite levels are almost 0, then add 2 or 3 small fish (depending on tehs ize of your tank that is, my 260litre tank i can add 10 small tetras in one go without any problems, in a 2ft tank this would be bad news).
When they have been in there around a month with no problems, then you can add a similar amount.
What you are suffering at the moment is called 'New Tank Syndrome' and its unbelievably common because shop owners do NOT tell people the truth!
The only way you can get away without setting up a tanka dn leaving it without fish for 6 weeks is by seeding it frmo someone elses tank (and is what i do when i set up a new tank), by using their old filter media and some of their old water.
Hth
Em