Soak tamarind in water for 10 mins. Mix well and squeeze the juice alone, set aside.
Heat 1 tblspn oil in a pan, add in onion, dry red chilli and coriander seeds, Roast till golden. Remove this to a blender and add in coconut. Add little water and make it into a fine puree.
Now in the same kadai. Add in mustard seeds and let them pop.
Add in onions and curry leaves, add some turmeric powder as well. Saute this till onions turns light golden.
Now add in tomatoes and cook till the tomatoes turn little mushy.
Add in the ground masala, water and tamarind water.
Bring this whole thing to a good boil, cover and simmer till oil floats on top.
Now add in fish and cook for 10 mins or till the fish is done,