Keep your terrarium in a bright room, north facing window is best.
If it is a hot day draw the blinds so the leaves do not get sun burnt during the heat of the day. Also take the lid off for 5min to allow it to breathe. Mist it lightly with fresh water before placing the lid back on.
Trim any dead leaves or remove them from the ground.
If your terrarium has no lid water it with a mister daily or pour water weekly (use a shot glass).
It's ok if little white bugs live in there, they are helping break down all the dead roots and leaves. If they were not in there you would have mould. and yuck!
GNATS - if you just have a few gnats flying around it's ok, they are not hurting anyone. But there are times when they can become a serious issue. Generally, the potting mix you buy will have some gnat eggs already in there, and given the correct conditions, warmth and moisture they will breed like rabbits... or gnats. You can try spaying them with pyrethrum bug spray, bug that won't get the eggs, if you have a really bad infestation.
Another alternative is to buy predator bugs, they will get in there and eat up the gnats and their eggs.
If you have any questions email