depends on how much your topping off and how big your tank is.ive did it before just because i ran out of ro water. also most krogers and publixs have ro machine for 29 cent gallon
i have a 5glaaon container that i put water in and some stuff that removes clorine and i just top off that way . i also have 125 and only put in 2 to 3 gallons at the time.