Congrats on your new dragon! I have 6 beardies :shock: - and we have a rule in our house. We don't ever switch tanks and down-size a dragon. Any changes to your bearded dragon's home is stressful. Beardies don't like their environment radically changed and I would hate for you to upset your own dragon when he isn't having any problems. This adult has been in this 20 gallon for a while and he will be okay for a little while until you can get him something bigger.
Another reason for not switching their tanks is that you don't want to expose your dragon to any diseases or parasites that your new dragon may carry. A one to three month quarantine of a new dragon is pretty standard for most people. I heard of someone who had a dragon die of adenovirus and weeks later had that old tank swabbed for the virus and it was still present in the tank. :shock:
If you are still wanting to switch tanks, disinfect the used tank with Wipe-out or some similar reptile-safe disinfectant.
You said your new dragon likes lettuce. What kind of lettuce are you feeding? The reason I ask is because lettuce has no nutritional value to bearded dragons. The best greens to feed are collard greens, turnip greens, mustard greens and kale. Here is a wonderful website with an awesome page on nutritional value of foods for your bearded dragon.
http://www.beautifuldragons.com/Nutrition.html
I'm trying to see your UV light in that tank and I may be looking at it wrong, but is there a plastic cover over the tube light? I think that is what I am seeing but I may be wrong. The plastic cover blocks the UV and your dragon won't get the UV that it needs. If you were given the UV bulb with the dragon, go ahead and replace it unless you know it is less than 6 months old. Tube UV bulbs stop giving off the proper UV at 6 months. We write on the calendar when we replace bulbs and go forward 6 months and write on the date it needs to be replaced. Repti-sun 10.0 is the best bulb out there. They even have a bulb registry on the Zoo-Med site that will remind you of when you need to replace your bulbs if you register them. Pretty handy!
Your dragon needs to be able to get within 6-8 inches of the UV light in order to receive the full effect. We have raised platforms and reptile hammocks in our vivs to allow our dragons to receive full UV and be the correct distance from them.
I hope this info helps and I hope your new dragon is doing well.
Sure is a pretty beardie