Check out these great tips I found online and great if you do a succulent Terrarium
Your terrariums will thrive best in bright, indirect sunlight. While sunlight helps color up your plants a bit (some turn bright yellow, some turn purple or even pink), too much direct sunlight is harmful. This is because your glass vessel essentially works like a magnifying glass, which will heat up your terrarium and cook whatever moisture is in your soil. This rapidly increases the humidity and temperature in your soil, reaching the perfect conditions for unwanted bugs and fungus to proliferate. Your best bet is to leave it on a desk or table in a room that gets natural sunlight or near a south-facing window. Be sure to rotate your vessels often to ensure sufficient and uniform lighting for your succulents. You can tell your plants are not getting enough light if they start to grow very tall with the leaves spaced apart. This phenomenon is known as etiolation, designed to help plants grow tall enough to gain some sun exposure. Remedy this by pruning your plant down to shape, using clean, sharp tools and let them get more sun.
The most important thing to remember about watering is don’t overwater. It’s simple. Try to minimize watering to something like once a week depending on the size of your terrarium. A rule of thumb is to stick your finger into the soil and if it doesn’t clump up, it’s dry enough to rewater. Water generously but don’t over do it. To give you some perspective, I set my squirt bottle to ‘stream’ and shoot once or twice at the base of each plant, then turn the bottle to “spray” and spray once over everything to give the terrarium a gentle misting. Watering too much will cause root rot, foul odors, fungal growth and a host of bugs! At least, that has been my experience so far. Also be sure to use lukewarm or warm distilled water. You can find distilled water or reverse-osmosis (RO) water at your local supermarket for very cheap. Avoid cold water and tap water, as your tap might be hard water. Hard water might be too basic (pH is unsuitably high) and it leaves mineral deposits on your glass and in your soil. Mineral deposits make it more difficult for your plants’ roots to absorb the water it needs, and are generally unsightly. It also obscures your view into the terr, preventing you from noticing any potential problems with your roots or soil.
After misting the vessel with RO water, the droplets help drag down dirt clumps stuck to the sides. If they persist, use a soft cloth to gently wipe the sides. Avoid using chemicals like Windex when cleaning your glass as they could damage your plants. Again, avoid hard water as the residual spots are difficult to remove.
Pests:In my experience, the most common pests I’ve encountered are fungal gnats and whiteflies. Fungal gnats occur from overmoistened soil serving as a substrate for fungus to grow (mycelial or cobweb, I forget). The fungus acts as a food source for fungal gnat larvae, and the moist soil serves as a convenient “nest” for eggs. The larvae eat roots and the adults swarm and spread to the rest of your house and other plants. Whiteflies appear as tiny white dots that crawl around, find a nice juicy leaf, and enter a stationary feeding phase during which they suck out the phloem of the plant’s leaves. This weakens the leaf and strengthens the larvae. They eventually secrete honeydew, a sweet, sticky liquid that coats the leaves and allows for fungal spores to attach and grow. Fungus that grows on leaves blocks the leaf from receiving adequate sunlight, turning the leaf yellow and sickly. The best solution here is prevention — don’t let your soil get too wet and too humid. If you experience fungus, cut down on watering and remove it from direct sunlight. You can use ground cinnamon powder as an organic fungicide. Its volatile oils kill the gnat larvae’s food source without harming the plants and can be safely washed away. Gnats and adult whiteflies can be trapped using yellow sticky posts. Whiteflies in general are highly resilient to pesticides, but can be controlled by direct contact with a dilute insecticidal soap. They live on the underside of plants so be sure to watch out for them!
At this point, you know practically everything I know about terrariums! I can’t guarantee that your terrarium will go perfectly the first time around, even if you follow these guidelines. I’ve gone through three iterations of my terrarium configurations and endured a lot of frustration. However, with each new terrarium I make, I learn something and get better at it. The results are more impressive and more satisfying each time. Do not be discouraged if this happens to you, simply build off of what you know and start over. A thriving terrarium is such a rewarding sight… I wish you all the best in your new, greener endeavours.