I know that siphoning removes water and cleans out waste from under the gravel. But in water change, you have to replace the water. However, I haven't gotten it to flow the opposite way. With a bucket of clean water, I placed the siphon in headfirst and the plastic pipe end into the tank. I tried starting the siphon the usual way- bring it out of water and put it back in once the water drains. It didn't work. I would have bubbles coming out from the tank end each time I tried to start, but that was the furthest I got. I tried to hold the bucket above the tank (there isn't any surface I could put it that would be above the tank, so I had to just hold.) No luck. In the end I gave up and did what I normally do- just pour in the water from the bucket directly into the tank.