I just sold the two 55g blue food-safe drums I bought from a guy in CT last year. These had vanilla syrup or something in them originally. They also had the 5g graduated marks.
I cut out the tops with a sawz-all and then bought some brute trash barrel lids (not cheap!).
I used one to hold fresh RODI water; had two little holes drilled near the top for my RODI float/backpressure switch and a 1/2" tube connected to a MJ600 for ATO use (along with the OSC command on my Apex).
The other had no extra holes and was used to mix water. When it was time to do a water change I drained my 100g stock tank down, added in new water, drained a little more down, and then more new water back to my normal operating line.