I don't really understand your question or statment DHEEPAN . As far as how many freshwater species , God only knows as there's only a couple that come into the trade . The remaining are brackish or saltwater species . The species I currently keep are Doryichthys martensii from Thailand. I have twelve in a twenty gallon long tank with ALOT of plants , strong filter , and weak current . They do fit comfortably in a twently gallon as every fish has their own unique personality and habits . Some will rest among the plants , some are on the surface , while others are face down like a blade of grass . Weekly water changes as well as weekly water testing is important IMO . Up till now , they have been feeding on mosquito larve they relish greatly . As winter has set in , I will convert them to eating brine shrimp until the following spring . I do not know my sex ratio of my group but I'm hoping one day , I will be able to breed them . These are one of the coolest and most bizarre " oddities " available in the fish trade .