The stock tank you are getting, or that rigid pond liner that's like 150 gallons? I think having a bottom drain would be nice, even if it's a smaller pond, but for the outdoor pond, you could get a larger, flexible drop in liner, so it's like a regular flexible pond liner, but it's pre-formed, so you don't have to cut it to size or fold it.
http://www.justliners.com/preformedliners.htm
You could use a traditional bottom drain with one of those,, where you cut a hole in the bottom of the pond liner, and the bottom drain has two pieces which screw together, and which sandwiches the liner between the two pieces. I also used some sealant where the bottom drain attached to the liner on my pond to prevent the possibility of leaks, but I didn't use a pre-formed pond liner. It's also good to use a felt underlayment below the liner, to prevent rocks from damaging it.