A smaller Rhinecanthus species like a Picasso trigger would do all right in a 48 gallon for a few years, since they are very slow growing and take at least seven years to reach their adult size. However, they do get big eventually, and triggers in general are not good community nor reef fish. If you think you might want butterflies or corals on down the line, I would rethink the trigger. Hawkfish, basslets, pygmy angels, gobies and jawfish can all make interesting additions to a mid-size tank. Have fun!