Because the safety is a timer. If the pump runs more than 1 min 45 seconds or 2 min 45 seconds continuous depending on what you set, it times out, shuts down and will never run again until you power cycle it, this is roughly equal to a 1/3rd or 2/3rd gallon top off. The Nano Osmolator is for small tanks, regular Osmolator is for bigger tanks, with more invested it makes sense to spend what basically is the cost one nice coral more for a safer top off system on a bigger tank. The full size has an additional safety sensor and is more precise and has a longer time limit.