18" is fine for a bearded dragon cage, as long as your heat light is not a UVB Mercury Vapour bulb. Also you don't want the basking spot to get too close, unless you use a 'cage' around the heat bulb to ensure your dragon can't jump up and accidentally come in contact with it.
Make sure you have a good digital thermometer with probe and/or a temp gun to measure the surface temps of the basking area.
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PHLdyPayne
Forum Princess

