Undo
Undo is strongest after one identifiable mistake, such as selecting a fish that opens a new unmatched species. It is less useful when several earlier taps created the problem.
Shuffle
Shuffle can expose new matches when the visible layer has stalled. Before using it, note your current pairs so you can recognize whether the new layout actually improves the board.
Extra slot or extra aquarium space
An extra space buys time; it does not solve the ordering problem by itself. Use it when one additional fish immediately completes a triple or exposes the final order item.
When to restart instead
Restart when the holding bar contains several unrelated singles and no near-term triple is visible. Saving a booster for a close finish is usually more efficient.