Siphons are made with priming bulbs so I suppose you could use one for emptying a tank. If you are using a gravel vacuum, they have a built-in check valve at the top of the plastic tube. By shaking the tube up and down a few times, water will start flowing without a priming bulb. (Each time you move the tube down more water slips past the check valve until there is enough water in the tube to start the siphon. If you don't mind getting your hand wet, submerge a lot of the gravel vac and its hose, then lift out the hose and place the end in your bucket. By submerging the hose, it will be full and the water can't drain back out of the hose because of the check valve. When the end gets dropped down to the bucket, the flow starts because the full hose is at a level below the tank's water line.