i have an 8L nano with 5 corals in it, and its fine, 3L weekly or so waterchanges no probs, a few snails to clean up and other random hitchhikers. only problem ive ever had is when a coral tried to spawn went a bit haywire but lots of big water changes and it was sweet. So if your careful and look at temp and salinity etc there not many issues other than maybe light levels needed for some corals. have a chat to john at organism as well he has a few good ideas as well as he also has one of similar size with around 12 frags.