Very helpful, thank you! It's a closed terrarium, so the airflow will always be very little. But I will try everything else :)

That is the real problem. It needs at least a small hole somewhere to allow excess moisture to evaporate and not condense on your glass. Otherwise you really do need to start everything sterile. Good luck with it.

Oh, I've not heard that before :/ I thought the whole point of closed terrariums was that they are air tight, & have their own little micro climate. I might see if I can find a workshop somewhere i think! Lol... thanks again :)

Well, they are a micro climate but that climate is also ideal for bacteria, mold, mildew ... even more so than for plants. In nature, /plants/ do not exist in closed up, sealed, damp climates with no air movement and no changes. Plant growth itself is much stimulated by changes. Wind, soil disturbances, variations in temperature, humidity, the change of the seasons. So there are definitely limits and drawbacks to an entirely sealed system. For the plants.

But if you sterilize everything before it goes into the terrarium, and if you get the moisture balance /just/ right, then you can do it. But first you need to make it sterile to kill the mold, mildew, and bacteria. I do this with all of my leaf baggies before I seal them up. We do it in tissue culture or flasking orchid seed. But if you get contaminates in there, they are also going to thrive and grow.

To sterilize soil you may spread it out on a cookie sheet and bake it in the oven at 350 degrees for 30 minutes. Or an hour if you did a thick batch. Careful of steam when you open the oven and make sure it cools down before using it. If you bake it with aluminum foil over top, it'll keep it fairly sterile and spore free until ready to use.

For plants you can give them (roots and all) a 5 minute soak in a 10% bleach solution, then rinse with distilled water. Test some plants first. It won't hurt most but some very tender varieties may object. Some people use a diluted alcohol bath or hydrogen peroxide but I do not know the dilution ratios off the top of my head. As these are standard phyto sanitary practices for plant material for bio-laboratories you can do internet searches for the ratios. It also works well for flasking seed.

The actual terrarium can be sprayed all over with isopropyl alcohol at a ratio of 70% or better. Bleach also works or running it through a heated dishwasher. Just be sure to put on fresh clean clothes right out of your hot drier and wash your hands, face, etc. I wear a mask if I'm working with tissue culture but it's not really necessary for setting up a terrarium. Still a good idea to spray your counter or working station down with isopropyl alcohol before you start and dip any tools in isopropyl alcohol as well.

Alternatively you can skip the sterilizing and go with a dry system. One that is not so humid and moist like a desert terrarium. Or just hope that you get lucky. But as condensation on the glass is not desirable, most people don't want a sealed system anyway. They'll want to be able to get in there and clean the glass, swap out plantings, remove dead flowers, etc. Or treat for insects, though making things sterile first will eliminate insect issues at least initially. You could always accidentally introduce mites on your fingers from your clothes after walking outside if you reach in there.

As Hyn pointed out, you have created a micro-environment -- one that happens to currently be ideal for certain fungi and bacteria. :-)

Self sustaining sealed terrs for more complex biota like multicellular plants and animals is actually very difficult. Just far too many variables to account for ... which is why efforts to do so are almost always dismal failures. The only verifiable attempts with some measure of success I know of are the little biospheres containing a couple tiny shrimp and a type of algae/water weed and even those do not typically last all that long in anything but a lab. Much easier and less frustrating to do an open or partially open terr.

Once it is sealed you wouldn't need to water or mist it, Gabriela. Even a zip sealed baggie, though far more gas permiable than glass, /can/ contain living plant material for more than a year without reopening it if it has a good seal and thick plastic. Leaves I seal up for propagation typically remain in the baggies for 3 or more months at a time without opening them up to add any water. But everything that goes in needs to be sterile or you'll get blooms of moss, fungus, bacteria, fungus gnats, etc.

Since it is a closed terrarium, the whole point it keeping it air tight. spraying anything could kill everything in there, and leaving it open to the outside defeats the purpose of it being closed. Completely sanitize everything (I usually bake my dirt in the oven at the highest temperature), and throroughly wash your hands with soap, and rinse all plants with water before putting them in. Fully closed terrariums must have something like springtails to eat the mould. Springtails are good because they eat the mould but not your other plants. I recommend adding these to all closed terrariums

throw in some springtails and isopods to deal with the mold and bacteria.