Well a 20g wouldn't last long enough for me to recommend that, especially a tall, lol.
A 65g will last quite awhile, though a 75g would really be a better growout.
The main thing with them is overall they're not as hardy as the yoyos and such, so you really really have to keep up on water changes with them and not let the nitrates get too high. They're also happier the larger the shoal, so though 5 is an ok number, many more would be better. The larger the tank the better because then you can have a proper shoal.