75 gallon aquarium--what watt light?

Hello all. I converted a 75 gal fish aquarium to a bearded dragon habitat for my daughter's beardie. He has become very lethargic the last week or so. I bought a new thermometer, digital with a probe, and a new 75 watt basking bulb. I also run a uvb light. Well i plugged in the thermometer and its only getting to 87 degrees.... so all this time when i thiught temp was 95-105, its been colder and now im afraid her beardie is sick. He is not eating and he is sleeping all the time. Even falling asleep in warm water.... i thiught maybe he was impacted. ... but that hasnt helped either. Vet cant see him till Tuesday. what watt temp would you reccomend? The dragon is almost two years old. We use a fake carpet (made for beardies) as a substrate. The tank is a 75 galon and the light is approx 12" from highest basking spot on wood log. Any thoughts on what i need to raise temp? Should i use a higher watt bulb???

For a tank that size I'd go with a 150w ZooMed Repti Basking spot lamp or a 90w (or so) PAR38 halogen flood light with lots of lumens (over 1700). You can put it on an adjustable lamp stand so you get the height just right for the temps. You could also put it on a lamp dimmer to cut a little bit of power if needed. Having the proper temps and a nice bright basking light should help. Having proper UVB exposure is critical as well. What are you using for a UVB light?

Its a zoomed LF 50

Any reccomendations for the right size uvb light for a 75 gal tank? Does the light have to cover the entire tank to provide health?

That's not really powerful enough for such a large tank. I'd get a T5 that runs about half the length of the tank (roughly) and mount it on the basking side so the basking site has nice intense UVB exposure. Here are some good options for UVB fixtures and bulbs:

This fixture http://www.amazon.com/Sunblaster-904296-NanoTech-Fixture-Reflector/dp/B00AKKUBDQ
With this bulb http://www.amazon.com/Zoo-Med-ReptiSun-10-0-Lamp/dp/B00AQU8HAO/

Or this combo set http://www.lightyourreptiles.com/22-t-5-ho-fixture-high-output-with-arcadia-d3-12-ho-bulb/

Thank you! I have ordered the light. Do you know if the bulb it comes with is a UVB bulb or do I need to purchase another one?

The ReptiSun 10 I linked to and the Arcadia 12% that comes with the kit from Lightyourreptiles are both very good UVB lights that will last for a year or so until they need to be replaced.