Even low-maintenance ecosystems need a little tuning now and then. If your terrarium looks more like a swamp or a science experiment than the lush, balanced world you had in mind, you're not alone.

Whether it’s persistent fog, strange white fuzz, or plants that seem to be slowly giving up, this article breaks down what’s likely going wrong—and how to bring things back into balance.

1. Fogged-up glass

Some fog is normal, especially in the morning or after misting. But when your glass stays constantly clouded and you can’t see inside, it's a sign the system is holding too much moisture.

Common causes

• Oversaturated substrate or drainage layer

• Poor airflow

• Wide temperature swings between day and night

What to do

• Crack the lid for an hour or two each day to release excess humidity

• Wipe down the inside glass if needed

• Use a turkey baster or pipette to remove standing water from the base

• Cut back on misting—many closed terrariums need less than you think

Tip: A hygrometer can help you track humidity. Aim for 70–90% in most closed systems.

2. Mold and mildew

A little mold is part of the natural cycle. But if it’s spreading fast, forming fuzzy colonies, or smothering your hardscape, it’s out of balance.

Common causes

• Excess moisture combined with stagnant air

• Organic material breaking down (like soggy leaves or wood)

• No cleanup crew to manage microflora

What to do

• Air out the container regularly to boost airflow

• Remove dead or decaying matter promptly

• Add springtails to consume excess organic waste and mold spores

• Gently scrub mold from hard surfaces with a soft brush or cloth

• Swap out any fully compromised components

Tip: Mold flare-ups are common in new builds and often settle as beneficial microbes take hold.

3. Dying or yellowing plants

If your plants are wilting, turning yellow, or looking sad overall, it’s a sign something in the environment isn’t right.

Common causes

• Overwatering or underwatering

• Inadequate or excessive lighting

• Wrong plant choices for your setup

• Substrate lacking nutrients or airflow

What to do

• Check the roots: mushy means too wet, brittle means too dry

• Adjust your watering schedule—many closed builds need very little after the first few weeks

• Match your lighting to your plants’ needs: tropical foliage = bright indirect light; moss = lower light

• Rotate your terrarium to balance growth and light exposure

• If your substrate is older than a year, consider refreshing or adding a top layer

Tip: Don’t be afraid to swap in new clippings. Healthy ecosystems are always evolving.

4. When it just feels off

Maybe the moss is dull. Maybe nothing’s growing. Maybe it just looks tired. This is the vague but familiar "something’s not right" phase.

Common causes

• Lighting that’s too weak or inconsistent

• Aged-out or compacted substrate

• No cleanup crew keeping the microflora in check

• Slow post-setup decline after the initial growth burst

What to do

• Upgrade or reposition your lighting to ensure consistent, indirect exposure

• Add springtails or isopods to restore microbial balance

• Prune old or leggy growth to encourage fresh sprouting

• Refresh moss or top layers to liven up the appearance

Tip: If it looks tired, it probably is. A few small changes can make a big difference without a full rebuild.

Final thoughts

Even the best terrariums go through weird phases. Small shifts in humidity, water, or light can tip things out of balance—but the good news is that most problems are easy to fix once you know what to look for.

Treat it like a living system, not a finished product. Every adjustment you make is part of a bigger learning process.