eggshells will add calcium (among other things) as it breaks down..
everything that happens in an ap system is a balancing act between the fish and the plants,, a "slow release" nutrient like eggshells are commonly used... another one is to bury a banana, or half a banana in your growbed - provided you have worms in there - to add potassium that will be slowly released as the worms and micro-organisms break it down
adding anything besides media and plants to a growbed has implications beyond your vegetables... that's all i'm sayin