Thanks everyone for your replies, I had no idea that stock tanks would shift like that when in the ground, good to know for the future.
However, the plan has once again changed! Husband says that he really wants me to have an awesome, decent sized pond for my fish and turtles, and that he really wants a rectangular one (as do I!) and that he didn't really care for the stock tank idea... so instead he has decided that my pond is going to be a birthday present for me and that we will get someone in to do it professionally and beautifully! That way I will get a good gallonage, and he will be happy that it's done by a pro and will look pretty. I am really excited by this! I was looking forward to building it myself but it's already gotten complicated and I have barely begun. This way, I can get to really enjoy it without the stress. I am still planning on keeping goldfish rather than koi (I rehomed the one I had) because of the bioload with the turtles - and I love shubunkins so much that I don't feel like I am losing out!
I'm about to contact some pond contractors and see what the estimates will be, but I am still hoping for 1,500 to 2,000 gallons. Because I want a simple liner and paved edge rather than natural look, I am hoping the cost will be a bit lower too.
Still hoping to integrate a bog pond into it, or perhaps a separate, partially buried trashcan filter idea with plants growing out of it so it's less obvious. I am still at stage 1 of a total redesign so I need to do more reading! Thought I would share the update so far though.
Also, if anyone lives in the Dallas area and has any recommendations for pond builders, I would be so happy to hear!