Prepare Base:
Combine the full 20 ounce can of crushed pineapple with its juice in a large mixing bowl. Do not drain as the juice is essential for activating the pudding mix. Immediately add both boxes of instant pistachio pudding powder and stir continuously for about 2 minutes until the mixture begins to thicken noticeably. The acid in the pineapple juice will react with the pudding creating a thicker consistency than you might expect.
Create The Fluff:
Gently fold in the completely thawed whipped topping using a rubber spatula with a light hand. Use a figure eight motion rather than stirring to maintain maximum fluffiness. Continue folding until no white streaks remain but avoid overmixing which can deflate the mixture. Add mini marshmallows and continue the gentle folding motion until evenly distributed throughout.
Add Texture:
Fold in the chopped pistachios with the same gentle technique used for the whipped topping. Reserve a small handful for garnishing later if desired. The mixture should now have a consistent pale green color with visible pieces of pineapple, marshmallow and pistachio throughout.
Chill And Serve:
Cover the bowl tightly with plastic wrap and refrigerate for at least one hour. This resting period allows the pudding to fully dissolve and the flavors to meld together. The texture will continue improving for up to 4 hours. Before serving use a large spoon to gently fluff the mixture and transfer to a decorative serving bowl. Garnish with additional chopped pistachios for visual appeal.