Am I stocked?

[FishFanaticSyd]

I have a 26 gallon marine tank that I am wondering whether I can add fish to or not. I am relatively new to saltwater aquaria and am not sure whether my tank is fully stocked. I have an undergravel filtration system combined with about 15 pounds of live rock. I have one Royal Gramma and a Percula Clownfish. Can I add any fish and if so, what kind?

 

[54seaweed]

possibly a sand sifting goby

 

[FishFanaticSyd]

Would I need sand for that fish? I have coral gravel.