people say to use twice the amount of water 1 gallon bag 3 gal water? well what if you got 36 1 gallon bags with dirt thats alot of water? so.... ? oh yeah did i fail to mention im in an apartment upstairs?YEE!!!
If you're talking about pre-harvest, unless you've seriously over-nuted your girls, it isn't needed. Otherwise, if you need to do it (salt build-up?) just take one at a time into the tub, flush, set her aside on a pile of towels to finish draining and do the next, and so on, until you're done.