Keep track of your tank levels with Manual float type level gauges. The manual float gauges easily install in any 2" top opening and give you instant level of your double walled tanks. Gauges are available in stainless steel/poly or all polyethylene models. Level listed in 1/8, 1/4, 3/8, 1/2, 5/8, 3/4 and full in easy to read format. This Level Gauge for 40 Gallon Gemini is for tanks 3901 and 3902.