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i would not change the temperature the best thing to do is quarentine your sick fish and treat him with some medication here is a goor article for you to read hope it helps good luck. http://www.bestfish.com/saltich.html
Raising temps does nothing for marine ich. All your fish need to be treated in a qt tank. puffers do not tolerate copper since they are scaleless you should look into hyposalinity treatment.