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Some uncommon brackish options: Hogchoker, sheepshead minnows, several species of killifish, If upgrading in the future is an option then Mono argentus/sebae work well, although they'll outgrow a 40g and eventually need to go to full SW.
Dragon goby grow too big for a footprint of 40g tall. Not recommended by me. Figure 8 with knight or bumblebee goby is my favorite. Those fish are easy to find in my area.