As long as the flow in the display is well matched to the flow capacity of the HOB overflow bubbles will not build up in the U tube. Without bubbles in the U tube the syphon is maintained. With the variable speed pumps it is easy to dial the flow to be just right to not be too fast and make a flushing noise but not so slow that bubbles build up. I turn my pump off 2Xs every day to feed and it starts up fine every time.Until the power goes out and your pump doesn't restart, nor does the siphon. Ask me how I know!