Two 50% water changes is better than most do, but the extent of how beneficial those changes are, has more to do with the volume of tank water, and size of fish.
i.e if its a 50 or 75 gallon tank, two 50% changes could easily not be enough for a medium oscar.
And also, the idea that "water parameters are fine" may vary aquarist to aquarist.
While we all might agree any ammonia is bad, to me, any nitrate concentration over 5ppm is not fine, so I try to do a water change if nitrates hit 5ppm