Whitespots?

[Mollydolly]

Hi i brought a new fish today its a platy and is black and has a white underbelly. I noticed tonight he is very quiet and hides amongst the plants and doesnt venture out.
He does this shaking shuddery thing and seems a bit withdrawn compared to the other new one i also brought at the same time.
My lights are on a timer and when they went out he seemed happier and came out for a small swim. Anyway i was looking at him and have noticed he has a white spot on his upper body area its not a marking. It looks similar to when scales are missing but only in that one tiny area. So in case its anything like white spot ive moved him into my hospital tank and hes alot happier but still hiding.
So my question is do i treat the whole tank he was in for an hour for white spot or just him. Hes staying in the other tank for now till im sure he is healthy.
I cant post a pic here as it wont upload but can try to upload on to photobucket.
How do i treat it ive read a few posts about it but still not sure. Hes now in the 27l tank with heater and filter and just some stones so he doesnt infect any plants or gravel.
Can anyone help please

 

[eaglesaquarium]

A pic will definitely help.
 
 
This doesn't sound like whitespot (aka ich).  But, information for the future... if you ever encounter ich in the future, don't bother to isolate the infected fish.  Ich is a parasite and it really infects the tank. 
 
 

 
 
ONLY the free swimming stage can be killed by the meds, so removing the fish doesn't really fix the problem.  The entire tank must be treated when ich strikes.  (And it will eventually... it almost always does eventually.  Usually riding on a new arrival, which is why a QT is so highly recommended.)