This is a discussion on hi within the Introduce Yourself forums, part of the Welcome to Tropical Fish Keeping category; -->
Hi im beweeb im 13 and i have two tanks a ten gallon and a 20 gallon
im thinking of turning my 10 gl ...
Most brackish fish get pretty big. The only brackish fish I'd consider keeping in a 10g would be some of the livebearers, such as guppies, endler's livebearers, and platies, although most of these are probably going to be easier found acclimated to freshwater and thus won't do so well in brackish environments. Another option is the bumblebee goby. Although they're small, they can be pretty territorial so I'm not sure you could keep more than two or three in a 10g.
Beweeb, cardinal tetras and plecos hail from Amazon where water is soft and acidic. They lack tolerance for the presence of salt so you really cannot keep them in brackish conditions otherwise they will suffer from osmotic shock due to very high conductivity level.
As far as brackish water, plenty of the common brackish water species do grow large. Bumblebee gobies can do well in it however it is not really needed. BBG's can thrive in freshwater equally just as the glassfish do although the glassfish is not exactly an option in a 10g. It is only a myth both require brackish conditions. I used to keep 4 Brachygobius aggregatus for 2-3 years myself in a FW tank. I simply caught them in my local rivers but that was because I live in Asia so it is possible.;)