No way on the GSP. Like PP said, they absolutely need 30 gallons each, and then what would you do with the other F8? Plus, GSPs go up to almost marine conditions as adults, which IMHO is harder than just light brackish and requires different equipment/filtration (protein skimmers, live rock, etc). The tank would be fine for 2 F8s as long as that leak can be fixed. I'd offer $30 since it is leaking. You never know, might get a good deal!
Otherwise, there's always Craigslist, newspapers, and even garage sales and thrift shops.