Chop the shrimp into bite sized pieces and put them in a non-reactive bowl. Juice the limes and lemons and pour the juice over the shrimp.
On a shallow plate, cut the grapefruit into small segments. Add the segments, along with any accumulated juices, into the shrimp. Stir in the minced serrano peppers. Cover and refrigerate at least 1 hour.
Before serving, stir in the avocado, olive oil, and cilantro. Season to taste with salt.