You can do both. Water changes for the water quality and treat the tank at the same time. If it is a treatment that has to be added every day, do the water change then add the med. If it isn't dosed every day, on dose days do the water change first, and on non-dose days just add enough med to replace what you remove. Daily 25% water changes should be enough to sort out water quality, so on non-dose days just add 25% of the dose after the water change.