You need whatever watt bulb is required to keep the basking spot at the right temperature. This will vary depending on the ambient temp of the room, the type of enclosure, etc.
do not use a fish tank for a chameleon.
Veiled/Chamaeleo:
day basking spot up to a max of 96F/36C (Non breeding females and Juveniles hottest spot should be kept at a lower temp, closer to 80F, and yours is not quite adult yet so perhaps around 85F)
day mid-cage / ambient range 70-80F/21 -26C
night to a low of 60F/15.5C (Juveniles only down to 68F)