When to replace bearded dragon UVB bulbs

Good instinct asking this early — invisible UVB degradation is exactly the kind of thing that quietly causes metabolic bone disease before owners realise anything is wrong. Bad lighting is the most common husbandry mistake with bearded dragons, and the insidious part is the bulb still glows long after the UVB output has dropped to useless levels. To give you accurate guidance, I need a few details: what size is your enclosure, which UVB bulb are you currently using (brand and wattage), and how old is your dragon?

Right, a couple of things worth knowing here. T8 tubes degrade significantly faster than T5 HO tubes — most T8 UVB bulbs, including that Repti-Zoo, should be replaced every 6 months maximum, even though they'll keep glowing for well over a year. At 4 months old your dragon is in a critical growth phase where D3 synthesis matters enormously, and a T8 at 10% in a 4x2x2 is already working at the lower end of what a juvenile needs. Can I ask — where is the tube mounted, and how far is your dragon basking from it on a typical day?

That distance is workable, but a T8 10% at 30-35cm is producing a UV Index of roughly 1-2 at best — a juvenile bearded dragon needs a UVI of 3-4 in the basking zone. My honest recommendation is to replace it now with an Arcadia T5 HO 12% Dragon Lamp. It's a 39W or 54W T5 tube depending on which kit you get, and at 30-35cm it delivers a UVI of around 3.5-4, which is exactly where a young dragon should be. The Arcadia also has a published lifespan of 12 months before significant degradation — so you replace it annually rather than every 6 months, which saves money long-term. Set a calendar reminder for 12 months from the date you install it, and don't go by whether it's still glowing.

You can buy a Solarmeter 6.5 UV Index meter — it's the gold standard and gives you a real UVI reading at basking height. It's around £200 though, so most home keepers don't own one. For a single enclosure, going by the manufacturer's replacement schedule is the practical approach. With the Arcadia T5 HO 12%, replace at 12 months. With any T8 budget tube, replace at 6 months. If you're ever in doubt and can't test, replace early — a new bulb costs far less than a vet visit for early-stage MBD.

You'll need a T5 controller and fitting — T5 and T8 tubes are not compatible with the same ballast. Arcadia sells the Pro T5 UVB Kit which includes the controller, reflector hood, and the 12% Dragon Lamp tube together, and it's the cleanest way to do it. The reflector alone increases UVB output by around 30% compared to a bare tube, so it's worth getting the full kit rather than piecing it together. Look for the Arcadia T5 Pro UVB Kit 39W on Amazon UK — it fits a 4x2x2 well and everything you need is in the box.