There is no point moving the cardinal tetra into a separate tank because the white spot is in the main tank now and all the fish need to be treated.
If you have an air operated filter (from an air pump), that should be sufficient.
If you have an internal power filter, raise it up so the outlet shoots across the surface of the water.
If you have a hang on the back (HOB) style power filter, reduce the water level by an inch so it splashes more.
If you have an external canister filter, have the outlet above the water surface so it splashes into the tank.
----------------------
The following link has information about white spot. Post #1 and post #16 are worth a read if you want to know more about the parasite.
This is a common question that is often asked, what is ich and how is it recognisable and what causes it? The real term is ICHTHYOPHTHIRIASIS. OR commonly known as white spot. It is an extremely comon parasite that affects aquarium fish. It is highly infectious and potentially lethal and...
www.fishforums.net