My tank varies from 80 at night up to 82 during the day. I keep my metal halide on for 15 hrs. Last month I had the temperature swing from 68 to 89 degrees for a day due to a faulty heater and some stupidity on my part
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I noticed the heater had quit in the morning and unplugged my fan to help the lights bring up the temperature since I my backup heater didn't work as well. When I got home from work the heat had gone up to 89. Everything pulled through it unharmed.
Anouther lesson learned!
Dave