Welcome to MFK, you'll have to forgive people for being a bit sceptical of anyone who signs up and starts talking about building/owning ridiculously large ponds for arapaima as 99% of the time they are 12 year old kids talking BS or dreaming of what they would do if they won the lottery.
It sounds like a very interesting [and ambitious!] plan and I wish you all the best. With regards to your motivation for breeding them, have you looked into the logistics and legalities of exporting them back to Brasil to "help declining stocks"? I imagine there would be a lot of hoops to jump through to make something like that happen, you can't just send them a box of fish to release into the Amazon. Also given the huge requirements for housing them, there is [or should be] a very limited demand for them in the aquarium trade. My 2c if you're interested in breeding a large South American bonytongue would be to set up a pool for a large group of black arowana, you've got a far higher chance of success IMO and you'll have no shortage of buyers for the offspring.
Regardless, keep us updated with your progress, there are plenty of knowledgeable folk here with experience setting up big tanks/ponds, and we're all looking forward to seeing the little pima grow!
