cant decide

A while ago I decided to make a refugium in one of the chambers on my Fluval Evo 13.5g, so I ordered a set of chamber inserts on Etsy. I ordered this specific one because it routes the flow through the entirety of each chamber as seen in the first pic, but I feel like I’m getting scammed at this...

So I was wondering, what should I get next fish wise. I have a 13 gallon with a pair of clowns and a cleaner shrimp + a clean up crew. I want to get a Exquisite Firefish, as my centerpiece fish. But I also want something that is productive for the fish tank like a sand sifting goby. What do you...