This paper shows ways to configure and optimize TELEMAC-3D, to get ready for the calibration of temperature stratigraphies in small lakes. Thermal stratifications are very sensitive to numerical instabilities and dispersion. Small lake models are additionally dominated by a high percentage of boundary nodes, a source of instabilities in numeric models. The small “Lake Monsterloch” (english: “Monsters Hole”) is a good prototype study for the many central european lakes, that suffered the same problems in the exceptional hot summers of the last decade. The lack of rain and river discharge, plus the long heat periods from 2016 to 2019 and the rising agriculture based fertilisers in the water reduced the dissolved oxygen to nothing. Without vertical mixing effects, the consequence is the dying flora and fauna as well as the resulting rot odour nuisance.