Fill the bottom of rhe tray with water, the added heat underneath will add humidity even if the domes don't seal. Low light helps keep the heat off the top of the domes. I usually vent my domes after a couple days.That's bad ass. Proper humidity management is why I got an aquarium. Used means cheap. The plastic trays and domes are garbage because they don't seal well enough to maintain humidity. If I still have problems, the next step is a heat mat. I'm pretty sure I still have one around here somewhere...