T5 vs Compact UVB: Which One Won't Fail Your Reptile

Every reptile starter kit ships with the same little coil bulb, and every experienced keeper eventually replaces it with a tube. This article is the explanation nobody includes in the box.

UVB is not mood lighting. It is how a reptile's skin makes vitamin D3, and D3 is how the body absorbs calcium. Too little UVB for too long and you get metabolic bone disease, the most common preventable illness in captive reptiles. So the fixture that delivers UVB is the one purchase where the cheap option can quietly cost you the animal.

The two shapes, and why shape matters

A T5 high output tube is a long fluorescent strip in a reflector fixture. Mounted along the enclosure ceiling, it lays a broad gradient of UVB across most of the floor, strongest under the basking zone and tapering toward the cool end. The animal moves through that gradient and doses itself, the same way it would move between sun and shade outdoors.

A compact coil is a small screw-in bulb, a point source. Its UVB lands in a tight cone directly beneath it and falls off steeply with distance and angle. In a tiny enclosure with a species that needs little UVB, that can be adequate. In a 4 foot desert setup it means one small hotspot of UVB in an enclosure the animal uses all of, and effectively none anywhere else.

That is the whole argument in two paragraphs: tubes light a territory, coils light a spot.

Match strength to species, not to the shelf

How much UVB a species needs tracks how it uses sun in the wild. The UV-Tool project, the reference zookeepers use, grades species by their basking behavior, from open-sun baskers down to shade dwellers, and recommends exposure ranges for each group.

In practical shopping terms:

• Open-sun desert baskers (bearded dragons, uromastyx): strong desert-grade tube, positioned per your setup guide. For a 3 foot enclosure the Arcadia Desert 12% ProT5 kit ($65 to $85) is the standard; a 4 foot build takes the 36 inch LumenIZE ProT5 ($110 to $130). Placement details are in the bearded dragon enclosure setup guide.
• Forest and partial-shade species (crested geckos, chameleons): a gentler forest-grade tube like the Arcadia Forest 6% ProT5 kit ($95 to $115), mounted so the animal can also get fully out of the light. Specifics live in the crested gecko and veiled chameleon setup guides.
• Crepuscular ground dwellers (leopard geckos): the low end of the forest range at generous distance, always with deep shade available. The leopard gecko enclosure setup guide has the layout that makes this work.
• Snakes, tortoises and everything else: the UV-Tool sorts every group into four Ferguson zones, with corn snakes and ball pythons in the partial-sun zone, tegus with the open-sun baskers, and uromastyx at the intense end. Find the animal's zone first, then pick the tube that delivers it at your basking distance.

The three rules that make any UVB fixture work

1. Distance is part of the dose. The same tube is strong at 10 inches and weak at 20. Set the basking platform at the distance your setup guide specifies, and remember screen mesh between bulb and animal cuts output substantially.
2. Replace on schedule, not on burnout. UVB output decays months before the visible light dies. A year-old bulb that "still works" is often decorative.
3. The animal needs an exit. UVB, like heat, is something the reptile should regulate by moving. A shaded end is not a luxury, it is half of the system.

The honest bottom line

If your enclosure is 3 feet or longer, or your species is a sun basker, the T5 tube is not an upgrade, it is the baseline, and the coil in the starter kit is the single most worthwhile thing to leave on the shelf. We put the tube in the same buy-once category as the enclosure itself in the bearded dragon shopping list, because replacing preventable vet visits with one good fixture is the best trade in this hobby.