Yes their most likely fungus gnats. Unless it’s the issue of leftover food sitting in the terrarium too long, they probably hitch a ride in with the any plant soil. It’s a good idea to take any plants that you plan on putting in terrariums and rinse the roots with warm water, you can even go as far as doing a 10-15% bleach water dip to help kill any other hitch hikers and rinse again, then repot in the substrate that’s going in to the bio active terrarium. I also will quarantine my plants in a sterlite tub with a gasketed lid for a couple weeks to be sure I don’t have other hitch hikers after the repotting to be sure I don’t introduce them to a bio active set up, if you have multiple set ups they may spread to the others. Almost all store bought (HomeDepot, Lowes, local garden center) plants will have fungus gnats nesting in the substrate.
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