Put the fish in a medium sized bowl and cover it with the lemon juice immediately. Carefully mix it to cover it well. Cover the bowl with cling film and chill it in the fridge until the fish has changed to a white colour, between 15 to 20 minutes. Take it out of the fridge and remove the excess lemon juice. Add the onion, cucumber, chilli, cilantro and salt, and mix them carefully. Serve chilled with corn chips (like Doritos).