Nurse sharks grow pretty quickly compared to other sharks. You'll need more very soon... much sooner than a year.
And they are not "FRIENDLY" by any means. Don't be fooled into a false sense of security with them. They account for a majority of shark attacks in the US. They are docile in nature b/c they are nocturnal. When people see them, they seem to just hang out on the bottom. However, your average nurse has a strike speed of about 9 milliseconds - and with as much force as about any 'big' shark.