I highly doubt that your tests would register anything related to the eggs/fry being eaten.and I thought the shrimp was feeding them it does take morsels of food round the back of the cave although I think it is hoping to save them for itself then again maybe its a payment thing going on and the clowns then tolerate the shrimp being in their area and to your test results question I'm thinking a nice fat feed on live anything would be detectable on nitrates? Although I guess the rock will take care of that as it gets higher but surely there would be some kind of lag before that happens?