Pot them in soil that is one part white silica sand to one part peat moss and cover the terrarium with a sheet of glass, allowing for air exchange. If this happens, cut back on watering and let the soil dry until it is barely moist. These carnivores grow in bogs and thrive in moist soil. But never give them tap water. The chemicals in it will kill them.
Light is also crucial.
Since fly traps grow where sunlight varies with the seasons, the amount of light these plants get should be varied accordingly. However, light and water are not enough to keep fly traps alive. They need to be fed. In summer they do best outdoors, where natural daylight yields the best plant growth and color. He recommends putting the plants on a table in a sunny spot in your garden in the spring after the risk of frost is over.