Well to start with if your tank is 55 gallons get some interesting fishes like Monodactylus kottelati,its a beautiful fish with a liquid silver body not unlike Monodactylus argentinus but unlike argentinus it doesnot lose colour after its fully matured and maxs out at 8 cms!!Also you could go for zebra blennys,and if possible Neovespicula depressiformes,the Indian Goblin fish.All of these are beautiful and unique as compared to other run of the mill brackish fishes.I would also suggest Canthigaster compressa.Its one of the more peaceful of the toby puffers and does real well in a brackish tank.Good luckchrisdef15;738915;738915 said:55 is way to small for a shark if that helps you make up your mind.