It's best to treat all the fish in the tank, not just the ones showing spots. Fish can carry the parasite without showing spots, and those fish can infect the others. Unless you treat the main tank, the infected fish can just be reinfected again.
If you do not have a heater, my suggestion is quick cure. It quickly cures them (hence the name
) and is not that harsh. Problem with it is that it may stain some decor a with a light blue hue, so you may want to remove decor during treatment. There are other ways to treat ich, such as salt, but I have no experience with them so I don't have anything to say about that. I only use heat anymore (without salt).
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