I have been told this tank was once upon a time a tidal pool saltwater setup but that was back in the 60's and 70's.
I guess Koi are cheap compared to the big saltwater fish I used to buy, and cheaper than a pair of good sized arowana and some rays! I don't have to start with a lot of big Koi- a bunch of smaller ones would be fine- they will grow pretty quickly in a 3500 gallon setup. The LFS has some really nice looking butterfly Koi about 6" long for $19.99.
Channel cats are $5 at Earl May, pacu can be found at petsmart and then you just add in over the course of say a year w/e you find appealing because you can fit just about anything.