You can buy huge round stock tanks at the farm stores, that hold around 4 or 500 gallons and cut out, say 12 x 24 holes in the sides and put in plexiglass windows every so often. Being small, they wouldn't have to be really thick, so they would be cheap. I don't have any idea what kind of filter it would take, maybe a big home made wet dry filter.
I wonder were one buys that much salt? Even the cheapest store's salt here is $45 a box for 200 gallons. That would add up fast with water changes. Still it looks like a really fun idea!