Depending on sandbed depth, the yellow watchman or shrimp goby is a great and hardy fish, but it requires a DSB
. Avoid the diamond watchman in this size of tank.
Depending on coral choice, you also may want to consider a yellow coral goby - the yellows are pretty hardy (if they eat), the greens and other colors not so much. However, if you plan on going SPS
, you may want to skip these, as they will nip at SPS
The firefish gobies are also nice top swimmers, but require a cover as they tend to go carpet surfing.
Some species of flasher wrasses are a good choice once the tank is established and has a nice pod population.