Leopard gecko UVB upgrade: coil to T5 tube

Switching from a coil UV bulb to a T5 tube is one of the best upgrades you can make for a leopard gecko — coil bulbs produce uneven UV output and have a long history of causing eye problems in reptiles. Reptile welfare is almost entirely determined by getting the environment right: temperature gradient, UV lighting, humidity, and substrate all need to match what the animal would experience in the wild, and getting any one of those wrong leads to chronic stress and illness. To give you the right setup advice — how old is your gecko, what size is the enclosure, and is this a new build or are you retrofitting an existing viv?

For a leopard gecko, 6% is the correct choice — not 12%. Leos are crepuscular and come from arid but not intensely sun-blasted environments. A 12% tube would push UVB levels far above what they'd naturally encounter, and over time that creates vitamin D toxicity risk rather than benefit. In a 4ft viv, you want the tube running along roughly two-thirds of the enclosure — not the full length — so she has a UV gradient and can self-regulate exposure. What's the height of the enclosure? That affects how far the tube needs to be mounted from the basking zone.

Perfect — that's helpful. With a solid top rather than a mesh lid, you'll be mounting the fixture inside the viv, which actually works in your favour because you get full UV output without mesh blocking it. Mesh can reduce UVB transmission by 30–50% depending on the gauge, so internal mounting with a 6% tube is the right call here. The tube should sit roughly 25–35cm above the basking spot when she's at her normal resting height. My clear recommendation is the Arcadia T5 Kit 6% UVB — the 39W 865mm size fits a 4ft viv neatly and comes with the reflector and controller included. Arcadia is the most consistent manufacturer for reptile UVB and their tubes hold output well across the full 12-month replacement cycle. Don't be tempted by cheaper unbranded T5 kits — UVB output isn't visible, so you have no way of knowing if a budget tube is actually performing.

A couple of things worth doing at the same time. First, ditch the coil fixture entirely — don't repurpose it as a heat source or anything else. Second, when you first install the T5, run it for 5–7 days at a slightly greater distance than your target height, then lower it gradually. Geckos that have had poor UV exposure can be sensitive to the step-up even at correct levels. Third, check your calcium supplementation routine — if she's been on the coil bulb for a while without effective UVB, her D3 synthesis may have been minimal and she's been relying entirely on dietary D3. Once the T5 is running properly, you can reduce the D3-included calcium dusting frequency and shift more toward plain calcium, since she'll be synthesising her own. Keep a note of when you install the tube — replace it at 12 months regardless of whether it's still producing visible light, because UVB output degrades well before the tube stops glowing.

Good choice. Once it's in, give her a week to settle into the new lighting pattern before making any other changes to the setup — you want to isolate any behavioural response to the UV upgrade specifically. She should start basking more deliberately and you may notice improved activity levels within a few weeks. Any questions as you go, come back.