SumpStage is a reef sump planning and commissioning tool for aquarium owners checking drain-down, freeboard, return flow, and ATO runway before plumbing or first fill. Drag the running waterline, adjust the return chamber, enter display and sump dimensions, then see estimated drain-down, empty freeboard, pump runway, effective return flow, and turnover in one local workflow. Use scenario comparison to review standard, high-flow, and conservative assumptions. Then log a controlled pump-off test, compare observed drain-down against the model, and work through a commissioning script before leaving the system unattended. SumpStage is designed for planning estimates. It does not sell equipment, recommend livestock care, connect to hardware, or replace manufacturer instructions. Always confirm with equipment guidance and a real pump-off test. Private by design: no account, backend, ads, tracking, analytics, or data collection.