1. Combine cayenne pepper, paprika, turmeric, garlic and ginger with 300 ml of water in a bowl. Grind the coriander and cumin seeds and add to the mixture.
2. Put the spice mixture into a pan and simmer on a medium to low heat for 10 minutes. When the sauce has reduced and thickened add the coconut milk, salt and tamarind paste and bring back to a simmer.
3. Add the fish. Continue to simmer, stirring occasionally, until just cooked through (about five minutes).
4. Serve with basmati rice, a vegetable curry and poppadams.