Im finding they can't decide if its in the flounder or sole family and that fisherman would throw them on the beaches and the pigs would choke on them. In the wild they can reach 6 inches so I'm not sure if they would work in your tank.
What makes you want a brackish tank?
Usually people choose them for the fish they want .Wondering what fish fit I wonder why brackish ?? Unless just concerned about 20g limits ..
I mean scats , archers and monos are out in a 20 due to size they can reach but most other brackish fish remain reasonable size ..
How about mudskippers ! Talk about unusual and needing brackish ? They need a land area though although something floating [like foam ] usually works ?
Ya so not sure about the flounder then. I wanted a brackish tank to try it out. But I didn’t want a bug tank since I already have three other tanks. I just wanted to test something new and if I love it I’ll get a bigger tank to meet requirements for other fish.