I had what seem to be fungus gnats that came in with my pond plants. I threw all plants out and thoroughly cleaned up any traces of soil. I still have a few flies (gnats), oddly, in the basement--plants were upstairs and no more gnats up there. Drains are clean and I never had them before in 15 years. We covered floor drain to be sure. We had a sump pump installed recently, but it’s completely sealed. Could eggs have been laid while the work was being done? There is NO moisture, food, soil in the basement. At my wit’s end!