DIY cold water pond using old bathtub

[Nephalem]

Hey all. Not sure if this is the right section for this, but I couldn't see anywhere else it would fit.

So my wife found this at her mothers this weekend.

http://i30.photobucket.com/albums/c3...0D0A0E270B.jpg
http://i30.photobucket.com/albums/c3...0E0A07269A.jpg

They were going to have it hauled off, but she was thinking about bringing it home and digging this patch of dirt out that is outside our front door and burying this in it, lining it somehow, and turning it into a really simple cold water garden pond. The dirt patch is an old flower bed that is run over with comfrey right now and it runs between our walkways so the size and location is perfect. Only thing is I don't know if this old tub is too small or exactly what to do to it. We've never set up a pond of our own. We have both helped family members build some from scratch but that is more time consuming and costly than we can deal with right now, so we thought maybe this would be a good way to simplify the process so we can try a beginner setup before going the full route in a few years.

The APROX measurements are as follows:
Outside edge 54"x54"x75")
Back tip to concave front <59")
Volume 51"x33"x16"
Total outer depth 23"

I tried to run those volume measurements through an aquarium volume calculator but it came up as like 119g? And that just sounds too big for a tub that looks so little.

I know that it's too shallow for koi. My wife was thinking fancy goldfish or something, but I haven't done any reading on them yet and I'm also not a huge fan of goldfish. Granted that's a biased view because I've never kept one "properly" so that kind of turned me off of them. What other species would you guys suggest for a smaller pond like this? We really love intelligent fish that learn to react to owners such as Discus, BGKs, Bettas, etc. So any cold water equivalant that has personallity and moderate smarts like them would be perfect.

Thanks in advance for any ideas/thoughts/comments/angry ramblings.