I have a 20 gal tank with 2 corydoras; 1 pleco; 4 neon tetras; 4 platies. I have religiously cleaned and changed 25%-35% water weekly. The water tests usually are very good except I have hard water and the pH is about 7.6-7.8. The aquarium is planted has been well established for 3 months. I quarantine new fish and I have had no new fish added within the past month. Yesterday I saw white spots on one of the tetras and sure enough I found that ich had spread throughout my tank to the other fish too. How could this have been introduced into my aquarium? I have researched so many articles and now I wonder if I am treating this appropriately as I lost a platy this morning. I started API Liquid Super Ick Cure last night at a 1/2 dose as the instructions say to do for sensitive fish like the corydoras. I turned the heat up to 82-84F, turned off the light, and covered the tank to keep it dark in there. My daughter wants to know what the odds are that any of them will live? Is there anything else I can do? What steps do I need to take to prevent this in the future? Is the hardness in my water an issue? I am very worried about my fish as this is the first time I have ever had a problem with them.