Cooked prawns? Really? With tartar sauce? LOL
My nems eat a mix of food. Lately it's mostly LRS Fish Frenzy as it contains fish, shrimp, scallops, mysis, and vitamins as well as probiotics. Other times it's fresh salmon or sushi grade tuna -- both cut into little chunks smaller than a pencil eraser (usally about half the size).
If you want it to grow, feed it a lot, possibly every other day depending on how well its eating. If you don't want it to grow, it'll be perfectly fine without any food -- living off of the nutrients provided by the symbiotic zooxanthellae algae that grows within the tissue of the BTA.