A Dragon fish? Would love to see a pic of it, there are many different kinds. If it is the violet goby then that can reach up to 24 inches, and may start ambushing your other fish, if not fed properly. And I believe dragon fish live in Brackish water, but assume can adjust to fresh/marine. Do you have a common pleco? Those can also reach over a foot long and both will need a much bigger tank in the future.
But nice tank otherwise!