Been toying with whether this would work or not, so after some feedback folks.
Make up a container of new salt water, and have the levels exactly the same between the tank and the new water. Then add two tubes which you first fully submerge to remove any air. Then place your thumb over the top of one end and pass them into the other container, and place one end in front of a powerhead to create a positive pressure.
The concept is the positive pressure pushes water slowly from the tank to the bucket, and the other tube will then siphon water back into the tank to maintain the level in the tank. Over time, all the water would mix evenly, then you just chuck out the bucket of water, knowing you have replaced a percentage of the main tank water without having to do much.
I'm thinking of doing this with my coral quarantine tank, which is about 20L, and using a 10L bucket, so that would mean changing about a third of the water using this method.