Huh? You are supposed to mix the fresh water with the saltwater. That is the whole point of topping off...
I believe his concern is the water pre-mixing while it is still in the bottle. Because of the difference in sg
, the salt in the water in the tank will diffuse up into the bottle, and lower the overall sg
in the tank system, which is not what you want. The ping pong ball will help prevent the pre-mixing problem. I think your theory will work just fine as long as the cage is narrow enough to keep the ball from being pushed sideways. I wouldn't worry about a gasket as long as the lip of the bottle is relatively smooth.