why do you say that? i have no idea on how salt water tanks work, but i sure would love to see something like that. You could almost make a tide pool of sorts (minus the tide)
I live in florida and when my dad was a kid he lived on anna maria island. He told me that were a couple of guys who thought it would be funny if they let a couple of juvi tarpon loose in a retention pond. Tarpon are able to live in fresh and they lived in the pond for years. My dad said he used to see them role and they lived off blue gill and tilapia.
biggest issue would be evaporation, you'd have to have a pretty slick top off system setup
i have to add 1-2 gal a day with only a small opened top sump