40 litre plus plastic storage containers can be used for a pr of bristlenose. Have an air operated sponge filter, some driftwood and plants in the tub. They will be fine. Just make sure the water doesn't get too warm in summer.
They can remain outdoors most of the year but it might get a bit cold in winter. If the water is above 16C they will be fine. If it drops below that, bring them indoors.