Its my understanding that the virus is specific to dwarf gouramis (and possibly other labyrinth fish) so your other fish won't catch it, unless they are of this species (or family). Also its a virus, so its possibly that it could stay in the water, though I can't find any information on how long. Its still possibly this isn't the case, just something to be aware of in case the other symptoms start to show.
As far as the test goes, I would hold it against the white card. Also, make sure you are following the directions perfect. You gotta shake the bejeezus out of the bottle and remember to let the tube set for 5 minutes before reading it.