Pour the coconut milk and stock or water into a heavy-based pan and bring to the boil. Reduce to simmer and allow to cook, stirring occasionally, until the milk thickens and reduces a little.
Stir in the red curry paste and cook for a further 2-3 minutes.
Add in the cucumber slices and continue to cook for 2 minutes.
Add in the fish and simmer very gently for 3-5 minutes. Very carefully stir in the lime juice and rind and a pinch of sugar. Be careful not to break up the pieces of fish. Taste and season with a little salt and black pepper.