The Complete UVB Guide: Who Needs It, How to Choose, When to Replace

UVB is the piece of equipment most likely to be installed and yet do nothing.

The tube is lit, everything looks fine, and the animal seems well — then six months later metabolic bone disease arrives. Here is the mechanism and the practice.

1. What UVB actually does

The chain works like this:

UVB on skin → vitamin D3 synthesised → D3 enables calcium uptake → healthy bone

Without UVB, the calcium the animal eats cannot be absorbed effectively. The result is not a slow onset of deficiency symptoms — it is an animal that develops MBD despite eating dusted food, which is exactly what confuses beginners.

MBD is irreversible: once bone has deformed, adding UVB afterwards will not undo it.

2. Which species need it

Required — without it they will develop MBD:

• Bearded dragons and other diurnal basking lizards
• All tortoises, including Hermann’s and sulcata
• Aquatic turtles such as the red-eared slider and musk turtle

Beneficial — they do fine on dusted D3, but condition improves with it:

• Leopard geckos, crested geckos and other crepuscular geckos
• Use low output (5.0 or below) and make sure there is enough shaded retreat

Not needed:

• Snakes (corn snakes, ball pythons)
• Most amphibians (pacman frogs, White’s tree frogs, axolotls)
• Birds and mammals, which get D3 from diet

3. Choosing: UVI matters more than the number on the box

The 5.0 / 10.0 marking refers to percentage UV output, but what determines the effect is the UVI (UV index) at the animal’s position — a function of both output and distance.

Rough targets:

Tubes versus bulbs:

• T5 tubes — stable output, wide coverage, the mainstream recommendation. Should span at least two-thirds of the enclosure
• T8 tubes — cheaper but lower output and faster decay; need to sit closer
• Compact spiral bulbs — narrow coverage, easily producing overexposure in one spot and zero elsewhere. Not recommended as a primary source
• Mercury vapour bulbs (MVB) — provide UVB and heat together, suited to large enclosures, but need adequate distance and cannot be used on a dimming thermostat

4. Three fatal mistakes

❌ Shining it through glass or plastic

Ordinary glass and acrylic block essentially all UVB. Which means:

• A lamp above a glass lid → useless
• Sunlight through a window → useless (visible light and heat pass, UVB does not)
• A fixture with a glass filter → useless

The lamp must shine directly, or through purpose-made metal mesh at most (mesh still cuts 30-40%, so allow for it when choosing output).

❌ Assuming a lit tube still emits UVB

The most insidious failure. Output decays with use:

• T5 tubes: replace at 12 months
• T8 tubes: replace at 6 months
• Mercury vapour: per manufacturer, usually 12 months

A tube can be emitting no useful UVB long before it stops lighting up. Practical fix: stick a dated label on the tube, or set a calendar reminder.

❌ Getting the distance wrong

Too far is the same as nothing; too close risks photodermatitis and eye damage. Follow the manufacturer’s distance guidance, and always provide a shaded area where the animal can escape UVB entirely so it can regulate its own exposure.

5. Other practical points

Photoperiod: mirror natural daylight, generally 10-12 hours on a timer. Shortening to 8-10 hours in winter is reasonable.

Keep UVB and heat separate in your planning: UVB tubes produce almost no heat, and the basking temperature comes from the basking lamp. Both should sit over the same area so the animal gets warmth and UV together — which is what basking behaviour is for.

Natural sunlight beats every lamp. Where practical, an hour or two outdoors in suitable weather does more for a tortoise than any fixture. It needs shade available, escape-proof containment, and supervision throughout.

Measuring: a UVI meter is the only way to confirm a tube is still working. They are not cheap, but for species heavily dependent on UVB (tortoises, bearded dragons) one is worth considering.

6. Recognising a UVB problem

Early signs are subtle, so look for them deliberately:

• Normal appetite but slow growth
• Hind-limb weakness, belly dragging when walking
• Slight tremors in the limbs
• Tortoise shell yielding under light pressure, edges curling up
• Lizard jaw softening, mouth not closing properly

Any of these means a vet, and simultaneously checking your UVB tube’s type, mounting distance and age.

Related

• Enclosure setup: heating, humidity and substrate
• Choosing heating equipment
• Calcium and vitamin supplementation