One year with monthly changes of 10% max essentially translates to a year without water changes. You absolutely need to start doing frequent and substantial changes but at first I might be inclined to only change about 30% of the water, and do so every day or every other day a few times. A year of accumulating nitrates and other toxins has likely created terrible water conditions...an occasional water test would have revealed this...and a sudden massive change, even with much better water, can be stressful.