I think 18ltr
there's 2 after each ro (yeah I think they are about 18/20ltr)
there's one in each van as well (just in case), they're a bit smaller - prob 13ltr.
have a small stock of them in the workshop as well - a 5ltr one, a black one (9ltr I think), a brodex inline one (same as Ionics but refillable - 8ltrs ish) and a beast of a vessel, not sure what size but its about 40ltr