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Maybe you do not even need a thermometer lol. The fish themselves are more accurate thermometers from what you told us.
It took me a day to raise the heater to 86F from 80F. I know fish would only adapt if the temperature change gradually. That was a while ago though. Ich never came back since they would not come out of nowhere.
The ich I got was from the fish shipped to me by Petsolutions.com, and it quickly spread to most of the fish from what I could see (probably all fish, but most ich symptoms hide under the gills)
The fish from liveaquaria.com were of higher quality with only 1 fish came with cotton like fungus on one side of its body and it died overnight.
Always use a quarantine tank for any new fish is the way to go.
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