Yeah Mat, especially with glass tanks cause of the open top. You need to add a little basking light to bring up the temps if your only using 100. I use 100 watt on my little 20 gallons and get 102 basking temp on a flat river rock. I also use 100 watts in my 4x2x2 melamine enclosures it all depends on the ventilation. You can also just lift up the basking site closer to the bulb if you don't want to add the extra basking light, or put a towel over part of the screen top this will hold in some heat. Just make sure you are still getting a good gradient. This is the most annoying part of setting up a tank, or changing to a different bulb. Just got to change the setup a bit and you should get it right.
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Mystical Dragons