each time you do a "just" 25% water change...
you leave 75% of the residual chemical waste products (nitrate) in the tank.
In nature, riverine species (like oscars), experience minimally, 100% water changes per day.
I have found that in my 300 gal system, (180gal tank, with 125 gal sump) a 30% water change every other day, and a heavily planted sump that consumes most residual nitrates is sufficient, and
I can keep most chronic diseases (such as HITH) at bay, without the use of 100s of $$$ in medications.
To me the question is, would you rather put in the time and care of proper husbandry?, or spend the time, stress, and money on meds?