freshwater or saltwater?

[Redtail....giant]

i have 40 gallon tank ready to be set up but dont know if i should go with freshwater or saltwater. I don't know if the tank is too small to house saltwater fish. Just curious what would u do/put in a 40 gallon.
so far i have these items
-canister filter
-volcano led light air pump
-sand gravel (i would really like black sand, if guys know where i could find it i would really appreciate it)

 

[Terrance]

what kind of fish do you want in it and how much more are you willing to spend on your aquarium? Its not a big tank, but its also not too small.

 

[Tiburon]

Drs Foster&Smith got's Tahitian Black Sand on sale.. $15 for
a 20# sack

 

[Redtail....giant]

looking for any kind of fish that will do good in a 40g, i would like it to be saltwater but most of the fish i like need at least a 100g tank. I'm willing to spend at least $500 but no more than $800.

 

[Redtail....giant]

Drs Foster&Smith got's Tahitian Black Sand on sale.. $15 for
a 20# sack

Thank you....i found what i was looking for.