The black wire seems to be the ground and the red lights I believe are indicator lights to let you know which tank is in use. It lookes as if the guy has wired the guages using the wires from the original guage and added the lights, not sure why, because if set up correctly the one gauge should read empty when the other tank is being used. Lookes like the wiring is the issue and I dont think your guases are ground properly or wired correctly. Disconnect the lights you won't need them and wire the gauges directly to the switch. Whenever you see the gauges dip the way the one on the left did when you turn the key it is because you have a grounding issue.