Place fish in a large non-metal, non-corrosive bowl (e.g. a glass or plastic mixing bowl). Mix in lemon/lime juice and leave to marinate on the bench for 15-20 minutes while you prepare the rest of the ingredients and the guacamole.
To make the guacamole, mash the avocado with a fork, and mix with lime juice. Season to taste with salt.
If using corn tacos, bake in the oven according to packet instructions, allow to cool slightly and then break into large pieces.
Add pineapple, red onion, chilli, coconut cream and coriander to marinated fish and miz gently to combine. Season to taste with salt.
To serve, divide marinated raw fish between small serving bowls and top with a dollop of guacamole. Serve with pieces of broken taco shell or corn chips on the side. It is easiest to eat this dish with a small spoon.