10 Lighting Mistakes Killing Your Reptile or Aquarium Plants (And How to Fix Them Fast)

Gabriel Anderson
Jan 27
4 min read

Split image of 10 aquarium plant lighting mistakes with solutions. Tanks show improper brightness, distance, and schedule issues. Text details fixes.

Many sick reptiles and dying aquarium plants trace back to a few repeatable lighting errors that are easy to fix once you know what to look for. This article walks through ten of the most common mistakes beginners make, plus fast, practical tweaks that protect animal health and instantly improve how the tank looks.

1. Using The Wrong Bulb For The Species

Many keepers grab whatever bulb the pet shop has on the shelf, without checking whether it matches a desert lizard, rainforest frog, low‑light snake, or shade‑loving plant.

Problems it causes

Reptiles that need strong UVB (dragons, tortoises, iguanas) get weak bulbs and slide toward metabolic bone disease.
Nocturnal snakes or geckos get blasted with desert‑strength UVB and develop eye irritation or stress.
High‑light aquarium plants sit under “decorative” white LEDs with almost no PAR, so growth stalls and leaves slowly melt.

How to fix it fast

Look up a care sheet and match the bulb type and UVB percentage (or PAR level) to the exact species you keep.
For reptiles, check whether your animal falls into a low, moderate, or high UVB group and choose a lamp specifically sold for that category.
For planted tanks, choose a light marketed as a “planted” or “high‑PAR” aquarium LED, not a generic decorator strip.

2. Putting Lights At The Wrong Distance

The same bulb can be perfect or useless depending on how far it sits from the animal or plants.

Problems it causes

UVB weakens drastically with distance, so a good reptile tube mounted too high gives almost no usable UV to the basking spot.
Compact UVB placed too close to a perch has caused eye and skin burns in reptiles in documented cases.
Aquarium lights too far above the water give leggy plants that stretch toward the surface and then fall apart.

How to fix it fast

Check the manufacturer’s chart for recommended distances and copy them; most give a safe range in centimeters or inches.
Use a UV meter for reptiles or a PAR map for aquarium LEDs if you want to be precise, especially with expensive animals or plants.
Raise or lower the fixture until basking perches or plant tops sit in the recommended zone.

3. Ignoring Bulb Age (Old Bulbs, New Problems)

Fluorescent and UVB bulbs keep glowing long after their useful output has faded.

Problems it causes

Reptiles under old UVB tubes stop making enough vitamin D3, leading to lethargy, poor appetite, and bone problems even though the lamp “still looks bright.”
Old aquarium bulbs shift spectrum and lose intensity, so plants yellow, algae takes over, and the tank gradually looks dull.

How to fix it fast

Write the installation date on every bulb or fixture with a marker.
Replace compact/T8 UVB and most plant tubes every 6–9 months; replace T5 UVB and quality LEDs on the schedule the brand recommends (often 12+ months for T5, several years for LEDs).
If animals suddenly decline and everything else is stable, suspect an expired bulb.

4. Running Lights Too Long (Or Not Long Enough)

Light duration is as important as intensity.

Problems it causes

“24/7” lights stop reptiles and fish from ever getting a dark period, disrupting hormones and sleep.
Over‑long photoperiods in planted tanks almost always explode into algae blooms.
Very short days reduce activity, appetite, and plant growth.

How to fix it fast

For most reptiles and community aquariums, set a timer for 10–12 hours of light per day, then complete darkness at night.
High‑tech planted tanks usually do best at 6–8 hours of strong light to avoid algae while still driving growth.
Use a cheap plug‑in timer so you never forget to switch lights on or off.

5. Scattering UVB, Heat, And Visible Light All Over The Enclosure

New reptile keepers often mount bulbs in different corners so the tank “looks nicer,” but that breaks the way reptiles use light in nature.

Problems it causes

The basking heat is on one side, UVB on another, and bright visible light elsewhere, so the reptile can never sit in one spot and get everything it needs.
Animals end up choosing heat without UVB, or UVB without heat, leading to either digestion problems or D3 deficiency.

How to fix it fast

Cluster the heat lamp, UVB tube, and bright visible light so they overlap at the main basking platform.
Leave the rest of the enclosure gradually cooler and dimmer, so the animal can move away to cool down and rest.

6. Blocking UVB With Glass, Plastic, Or Thick Mesh

UVB does not pass well through common materials.

Problems it causes

Placing UVB bulbs on top of glass lids or thick acrylic tops removes almost all useful UVB before it reaches the reptile.
Dense lids and floating plants on aquariums can block a surprising amount of light, starving lower plants of energy.

How to fix it fast

For reptiles, mount UVB above fine mesh or inside the enclosure where it cannot be reached but is not blocked by glass.
For aquariums, use open‑top tanks, mesh covers, or transparent covers only where necessary, and trim floating plants so light reaches down to the substrate.

7. Using Colored Night Bulbs Or Constant “Moonlights”

Red, blue, or black “night bulbs” are heavily marketed but rarely necessary.

Problems it causes

Many reptiles and fish can still see red and blue light, so “night” bulbs keep them in a permanent twilight, reducing sleep and raising stress.
Extra heat‑emitting colored bulbs can dry out enclosures or overheat tanks overnight.

1. Using The Wrong Bulb For The Species

2. Putting Lights At The Wrong Distance

3. Ignoring Bulb Age (Old Bulbs, New Problems)

4. Running Lights Too Long (Or Not Long Enough)

5. Scattering UVB, Heat, And Visible Light All Over The Enclosure

6. Blocking UVB With Glass, Plastic, Or Thick Mesh

7. Using Colored Night Bulbs Or Constant “Moonlights”

References