A siphon works by gravity. Yes, gravity can be used to push water up over the edge of your fish tank.

Gravity is pulling down on all the water in the fish tank. If there were a drain in the bottom of the fish tank and you opened the drain, the water would all come out, right? The siphon opens that drain.

So, to start your siphon you're really just getting water in to the siphon tube and up over that top point. Once you get the water started over the highest point, gravity will do the rest.

But that only works for the little bit that came over the top, right?

Well, sort of. Remember I mentioned gravity up above, and pointed out that gravity is pulling all the water in the fish tank down? Well, all that weight of fish tank water is pushing not just down, but also out the little siphon tube. All that weight of aquarium water pushing out on that little tube opening is enough to push the water a little higher up the tube.

At the same time, the water that made it over the high point in the siphon wants to just flow down the hill. The fact that this is flowing down also contributes a little to pulling more water over the high point, because, otherwise there'd be a vacuum, and nature abhors a vacuum.

This way, water will continue to flow, even though it has to go up hill a little, as long as the bucket end of the siphon is lower than the surface of the water in the fish tank, the tube is unobstructed, and the end of the siphon in the fish tank is below the water level...