65 gallon clear mixing vats left for TMP salt, right for RODI (doubles as ATO container- has 3 tunze top off pumps inside). i purchased online, delivered (a place off the 60fwy sells these as well). Ive used brute trash cans, sealed top blue barrels, but its helpful to see the level at a glance how much you have- ive been caught off guard before not having salt water for an acclimation or the top off being dry.
to finish the answer: the RODI container strictly gets no chemicals (not even buffer or alk) so it stays clean. the salt mix vat does build up brown crap on the walls but it always test free of PO4. ive poured some of the big vinegar jugs from costco through it and reused the acetic acid to clean powerheads, but residue has never been a problem.