I would only offer exceptionally large or difficult species as special order. Because what would I do with them if they didn't sell? Like thanks but almost no one needs an arowana.
I wouldn't carry anything as a normal stock item that required over 100gallons.
I would refuse to sell fish to people unless their tank was x size.
.... I would go out of business quickly :P
Yeh I would do this with bettas too. They would be ordered on request.