For tropical terrariums consider Hydro Drain. It is a special clay substrate which is excellent for water storage.
Hydro Drain acts as drainage layer below the normal substrate. The water seeps through the substrate and is stored in the Hydro Drain clay pieces. Once the storage capability is surpassed , you can still have water standing at the vivarium floor. As long as the fill height of Hydro Drain is not surpassed, the regular substrate will not get swampy.
Thanks to its water storage abilities Hydro Drain increases the humidity inside the vivarium and is excellent to supply plants with water. We recommend to add a minimum layer of 3 cm Hydro Drain and cover it with Hydro Fleece. On top of this the normal substrate is placed. The Hydro Fleece prevents substrate from seeping into the drainage layer but lets through the water.