It's ich and it should be treated in the main tank since the parasite has a life cycle which involves falling off the infected fish and laying eggs in the substrate. Increasing the temp will speed up the life cycle of the ich and helps rid the tank instead of having unhatched eggs that will start the cycle all over again. Salt has been helpful in causing parasites to absorb water until they burst. However, ich starts from the inside of the fish and works it's way out of a cycst, which is the white spots you see. You should pick up a general ich treatment, but read carefully as many tetras a sensitive to medications and many recommend half dosage for tetras and scaleless fish such as catfish.