The answer to 'supply line constantly running' is that a constantly running supply line is usually caused by a problem with the toilet fill valve or a leak in the supply line. This can waste a significant amount of water and increase your utility bills.
Common symptoms include the sound of running water, water pooling around the toilet base, or the toilet tank continuously refilling. Solutions may involve adjusting or replacing the fill valve, tightening connections, or repairing leaks. If you're not comfortable doing the repairs yourself, it's best to call a professional plumber to diagnose and fix the issue.