Soak Tamarind in hot water.
Chop small onions. Small onion adds flavour to tamarind curry, also don't compromise on the quantity. The more the tastier.
In a pan, add tamarind water (nearly 2-3 cups/ 2 tbsp tamarind paste in 2 cups water), 3 tbsp All purpose Chilli Powder (Kuzhambu milagai powder), ½ tsp turmeric powder, salt and 3 tbsp gingelly oil. Mix it properly.
Cover and cook for about 30 min.
Now the onions and tamarind would have cooked and blended properly.
Check for salt and hotness and add fish pieces.
Let it cook covered. After a few minutes carefully flip the fishes, for it to cook on the other side.
When its cooked, switch off flame and let it rest covered for sometime before serving. This curry tastes good with time. So prepare it early and give some good resting time for the flavors to infuse.