Good luck breeding them, they get over 2' in length.
I hear one of the key ingredients to keeping them healthy, is a maximum amount of oxygenation. Have your tank very well aerated, as the payara usually predate within the rapids of the amazon in the wild.. They are used to very pristine water conditions, which have to be very consistant. They need high levels of dissolved oxygen, but make sure the current isn't blowing them around the tank and they have spots for refuge at night. They are very sensative to swings in water conditions, so be consistant in your water changes as well.. No tankmates that could possibly chase, nip, harass, or stress them out..