wolfchan, there's no good substitute for natural sunlight. If your UVB light is more than 4-5 months old it could probably do with replacing, but even with brand new lights most beardies respond much better to sunlight than any so-called "full spectrum" bulb. There are wavelengths in sunlight that are good for psychological health as well as vitamin formation and the UVB bulbs don't necessarily provide that.
You're very lucky to live where you can give this sweet boy some sun in the winter and I'd say go for it as often as possible! Your basking spot can go as high as 115-120 and I'd say try bumping that up nice and high. Once you see him gaping and moving away from the hot spot, then bring it down a few degrees until you see him spending lots of happy time in the warm spot. A rheostat or dimmer switch is very useful in times like this. Some folks also say a higher watt bulb dimmed back some seems to last much longer than a lower wattage burning at full strength. Unless you are using a mercury vapor bulb, they cannot be dimmed. You want his digestion to really get a kick start and nothing like a good hot basking spot to do the trick, especially if he can also get natural sunlight regularly.
Do you have any small rubbermaids you could use as a feeding tank? I often use sweaterboxes as feeding stations with my hatchlings and rescues so that they don't have a big trek to hunt down their prey and you don't have the stress of too much handling at feeding time. That might help him catch more bugs at a time. Silkworks are an excellent food source, much better than waxies which are mostly fat and not much nutrition. I'd use waxies just as a treat.
Great to hear about the greens! They will really help bring up the hydration status and provide some good nutrients too. See if you can get some orange squash into him as well, it's a big nutrition hit also and most beardies I've met love orange foods on sight. Many tongue-flick my nails if I'm wearing a nail polish that's kind of orangey.
It's a hard call about the bigger tank. If he's doing well in a smaller feeding station I'd say give him the bigger space so that he has the least amount of stress and maximum potential for good thermoregulation and exercise. That's just my gut feeling, though. There are some animals that really need a smaller, darker cage while rehabbing or they don't feed well. You'd be the best judge based on his feeding, basking and hiding patterns. If you can find a small enough rubbermaid, you can put his feeding station right inside his tank so he feels less stress.
I'm glad to hear he tolerates soaking well, he'll absorb a decent amount of water even if he isn't a big fan of drinking. It can literally take months for sunken fat pads to fill back in if dehydration was bad enough to compromise kidney function, but most times it reverses within a few weeks in my experience. If the fat pads are very slow to fill out, blood work from a vet to assess kidney function is very important for his long-term health. He may need to go to a low protein, almost all salad diet to avoid stressing the kidneys. If he starts losing weight, goes off food, or refuses to bask get the blood drawn immediately otherwise you can probably hold off and see how time and fluids work. Blood draw and vet visits are so stressful that it would probably set his recovery back a few days at least.
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