A 75 gallon tank is really large. There are lots of big fish that you can keep in there. I agree with the above posters that a koi isn't one that you can keep in the tank forever (it will eventually outgrow the tank and need to go to a pond), but there are lots of really nice fish that you can put in a 75 gallon tank. Can I share my fish wishlist with you? These are fish that, if I had a 75 gallon tank, I would totally get. There are two different designs that I would love to have.
1. A planted tank with sunfish in it, maybe orange spotted sunfish or longear sunfish.
Image of orangespotted sunfish: http://www.epa.gov/bioiweb1/images/fish ... 1_Jeff.jpg
Image of longear sunfish: http://www.ux1.eiu.edu/~cfruf/images/bi ... unfish.jpg
They're so pretty! If I had a free 75 gallon tank, I would plant it with lots of live plants and then like a dozen sunfish. It would be so pretty :)
I mean, look at this bluespotted sunfish: http://gallery.nanfa.org/v/members/rick ... 1.jpg.html
and this dollar sunfish: http://gallery.nanfa.org/v/members/Will ... 5.JPG.html
2. A fast flowing stream tank with rainbow and yellowfin shiners in it.
Image of rainbow shiners: http://gallery.nanfa.org/v/members/farm ... 1.jpg.html
and http://lh3.ggpht.com/_hvRfkrhCEyQ/SG858 ... 1.1%29.jpg
Image of yellowfin shiners: http://gallery.nanfa.org/v/members/Mich ... M.jpg.html
They're so pretty! A stream tank setup with a powerhead pushing water really fast, with the fish swimming in the current, loving it, that would be a great setup. Here's a sketch drawing: http://www.nanfa.org/articles/images/frontb.gif
Here's an example setup: http://www.youtube.com/watch?v=rf0qGiimtTE
And you could add rainbow darters to the tank, here's what a rainbow darter looks like: http://1.bp.blogspot.com/_HaJp5gHFhss/S ... re+189.jpg
Oh, I would love another big tank to be able to create one of those two setups. I mean, there are so many cool fish to keep. I have too many tanks already, though. But you're lucky, you've got a whole 75 gallon tank to work with :)