Paroon Sharks, related to Iridescents and prob'ly what that 6 footer was, can get up to 10'. Both are in the same family as the Mekong Giant catfish that's currently the largest catfish (possibly the largest freshwater fish). None of them are aquarium fish. Iridescents less so than most because, in addition to getting 4' long, they're schooling fish, so you're not looking at a tank, you're looking at a swimming pool.
That said, a school of 8 or 10 of those in a nice 5,000 gallon pond, maybe with a big ol red tail lurking at the bottom would be darn cool.