In a large glass bowl, toss together the tilapia, tomato, onion, cilantro, and peppers.
In a medium bowl, combine the lime and lemon juice. Pour over the tilapia, making sure all of the tilapia is covered with juice. Stir to combine.
Cover and refrigerate for at least 2 hours and up to 24 hours. Add salt to taste.
Use a slotted spoon to divide among the tostadas. Serve with sliced avocado, if using.
Notes
*Pro-tip: If the tilapia is too soft to cut, partially freeze the fish (check every 15 minutes; DO NOT completely freeze) before cutting with a sharp knife.