Fungus gnats aren't harmful per se, but they are symptomatic of cultural conditions that almost always are harmful. Soggy soils with rotting vegetative matter are a fungus gnat's delight. Soils that drain well and a lighter hand on the watering can are generally enough to keep them in check.

Al

Cacti and succulents aren't the usual denizens of a terrarium because those plants don't need the increased humidity, and usually need more direct sun than a terrarium can usually offer. There seems to be a fad lately to arrange them this way. I might encourage you to find a more appropriate planter for the c/s, and a diff group of moisture/humidity loving plants for the terrarium.

Since the terrarium probably has no way to drain excess water, one should get more longevity out of a planting in one if not using tap water which has various chemicals that can build-up in soil over time to toxic levels. Distilled, rain, and condensate from dehumidifier or A/C are sources of water with no chemicals added.

In addition to the cultural info above, if additional help is needed to battle fungus gnats in any kind of pot, you can soak mosquito dunks in the water given to plants, and/or use Bt on the soil surface.

This post was edited by purpleinopp on Tue, Sep 2, 14 at 7:32