i agree with both jones and gourami on this one
depending on the stocking levels, maintenance, tankmates and experience of the aquarist, a 6inch fish can live happily in a 20g, however, the *ideal* would be no smaller than a 45g. and a tank being of the *long* version is always preferable, but a nice air pump and long bubble strip can compensate for the oxygen exchange needed for a healthy system. (tho personally i feel EVERY system should have this)
Sobe, if you are going to be the main fishkeeper of your mom's tank, I'd say a school of 6-8 tiger barbs and a redtailed would be fine
if, however, your mom will be the sole source of maintenance, I'd try to convince her of a different setup.
either way, she will need to learn, right?
