I had 2 250 W MH bulbs over my 90 when it was running. In the winter, the ground floor would hover around 18-19 degrees and the lights alone could keep the water at a steady 24-25 during the day. In the summer we let the house heat up to 24 in the day when we're not home and I'd usually have to turn the MH off when I got home from work around 5:30 (3.5 hours early) because the tank would be pushing towards 28 degrees.
The tank had no canopy and was free-standing, so MH can definitely make a big impact on the temp of your water.
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