What causes the level in the overflow to fluctuate? I have a innovative marine nuvo 113int lagoon that has a bean animal overflow. The water level in the overflow fluctuates with the return chamber in the sump. If the return chamber is low due to evaporated water, the overflow chamber is also low. The problem with that is when i top off the water to the marked line in the return chamber, for what ever reason over time the salinity is creeping up like i am not adding enough water. How do i correct this? i don't have a auto topoff and was planning to start off by doing manual top off as i really hate auto top off systems.