Are you using fem seeds. If not I'd suggest using one plant per tub because if you have to chop down a male thats in a tub with a female the roots will be tangled and trying to remove the males root ball could cause damage to the females root system, while leaving the root ball of the male after chopping it seems the logical solution it will only cause the males roots to rot out the females.. so if there feminized beans then I'd say 2 per tub should be good but 21 gallons might be overkill for one plant unless you vegged it aloooooooooong time... just my 2 honest abe's

later D:S5: