If several fish have multiple white spots (ich) it has been a few days. It is generally believed that ich may be present in most tanks permanently, but healthy fish will build up a resistance/immunity, for lack of better words. But severe stress triggers an outbreak. One thing is a certainty: ich is directly caused by stress if the parasite is present.
There is no point in removing any fish (unless you don't want them) as the ich multiplies extraordinarily rapidly and for all you know all fish could be carrying it. The gills are the first area attacked by ich, and the spots are not externally visible; signs of flashing usually occur long before any external spots can be seen elsewhere on the fish. Healthy fish not under stress may flash but that is as far as it goes.
The most effective treatment is salt and heat; this was brought home to me by Dr. Neale Monks. A few fish are unable to tolerate either the salt or the increased heat, but for most this is the preferred method, and definitely the most effective even in stubborn cases. I have treated some quite sensitive wild caught fish using salt and increased temperature, including cories, characins and loaches.
You mention the pleco and cardinal tetra...are there any other fish species? If not, I recommend the following, which is the method given to me by Dr. Monks. Raising the temperature has the effect of increasing the life cycle of ich, and it can even be killed at 90F/32C if the species can tolerate this but I needn't go into this as you cannot increase temperature.
Do a major water change, around 60-70% (assuming parameters between tap and tank water are the same or very close). Add salt at the level of 2 grams per liter; one level teaspoon of dry salt is approximately 6 grams, so this treats 3 liters of tank water. When calculating the tank volume, allow for displacement by substrate and decor, and underestimate rather than over. Dissolve the salt completely in a jar of water and then pour it in; never add salt crystals directly as they can burn fish, plants and invertebrates. Add the dissolved salt slowly, over a few hours. At the end of the first week (seven days), do a partial water change of 50%, add sufficient salt for only the replacement water. At the end of the second week, do another 50% water change but do not add salt. The rest will be removed with future water changes. One week can sometimes work, but as you cannot raise the temperature I would go for two weeks.
Salt is dangerous to all freshwater fish, no argument, which is why it should only be used as a specific treatment, and with fish less likely to have problems. The level suggested is sufficient to deal with ich and velvet as these two parasites are very similar. Other medications are usually much more dangerous, if they even work.
