For a start, they don't come cheap & your water has to right in all aspect.
A minimum tank size 5ft x 2ft x 2ft. The larger the better.
Good filter system, with somewhat strong current.
Water PH, to be hovering around 6.5 - 7.4.
Everything else 0.
They feed on small amounts of food regularly during the day. However, conditioning can be done.
Leopoldi will grow to 24" quite quickly and 30" in a few years. A 60"x24" tank would be fine for a few juveniles, but plan ahead for an indoor pond if you're serious.
I've got a pair of 10" P. humerosa in a 96"x24" tank and to be honest, they could stand a bigger setup.
They are not as hard to keep as many would have you beleive, you just need to be ready for that kind of commitment and prepare the equipment before you jump in. Think of them as giant horizontal discus and you'll be just fine.
Maybe check out Flat-fish.com for more specific information. (Sorry to the mods if linking is frowned upon.)