I currently have two marbled gobies in my Violet Goby tank. The SG is 1.008. I can't find any info on the SG required by marbled gobies and in fact most sites list them as fresh water. I can easily put them in a fresh water tank but I bought them with the intentions of being in my brackish tank. Currently they are 2.5" and 3". And yes, I know they get up around 20" when fully grown.