I give my fish a weekly treat of 150 ghost shrimp on a saturday afternoon, they love them.
Foods like lettuce, cucumber and courgette should not be fed as treats but as a daily staple food for herbivors and omnivors and frozen bloodworms should be fed at least every other day for omnivors and small carnivors. Weekly treats should be something live that the fish will enjoy chasing around and will be good for them too, daphnia, brineshrimp and daphnia are all good treat foods.