Here are a few tips on how to siphon without getting a mouthful of petrol or smelly fish-tank water. To siphon from an open container such as a fish tank, coil a few turns of hosepipe inside the tank (under the water), close one end with a fingertip, then uncoil it and remove your finger from the hose; the water will now run out of the hose.
To siphon from a petrol tank or any other closed container, insert the hose until one end is under the petrol. Wrap a damp rag around the hose at the mouth of the tank or container, push the rag tightly against the mouth, then blow (don’t suck!) on the other end of the hose. The pressure in the tank will push out enough petrol to start the siphoning process.