1.Chilli powder can be replaced with any homemade gravy powder/Kuzhambu milagaai thool(
link). In such case, reduce coriander powder to 1 tbsp as gravy powder already has coriander powder. But still, adding more coriander powder is to thicken the gravy.
2.Any fish variety will suit this curry recipe. Go for oily fish like Bronzini or Hilsa for more taste. Salmon too tastes good in this recipe.
3.Increase or decrease chilli powder as per your spice level. Pepper powder can be added at the end to increase the spiciness.
4.Adding fenugreek seeds powder at the end is optional but it elevates the aroma of the curry and also subsides bad odor from fish, if any. If you want to skip fenugreek seeds powder, then you shall increase fenugreek seeds to ¼ tsp, while tempering.
5.If you wish to avoid coconut, then grind 6 small onions and ½ big onion to a paste and add it once chopped onions are fried well. Sauté this onion paste until raw smell leaves and then add tomato. Add either coconut paste, or onions paste to give body to the gravy.