So I know this is likely a no from what i know of stocking however, im consulting the gurus to make sure. I currently have a 20g long display with a sump that holds at least another 10g for a total system volume of around 30g. The tank is way over filtered with the fuge and skimmer. At any rate its a mixed reef with one clown fish, one starry blennie, one cherub angelfish, and one royal gramma. I had the idea of adding a cardinal fish? Guessing im fully stocked and this is not a great idea.