To determine the liters per hour for a gravity-fed aquarium system, direct measurement is essential for accuracy. Users suggest filling the top vessel and measuring the water level change in the bottom vessel over time. A reliable method for maintaining water levels involves using a float switch to control a pump, ensuring consistent flow rates. Calculating flow rates with formulas is discouraged due to potential inaccuracies that can lead to overflow issues. Observing practices from tropical fish stores can provide additional insights into managing water flow effectively.