[Bombalurina]

I just want to add to what KoiMaiden has said. Your water change schedule is wonderful for keeping the water clean, but may be stressing him out. I'd suggest dropping down to 50% once or twice a week, and 100% at the end of the week. It will keep the water clean with less stress for your little guy. Stress contributes to sickness, so less stress = a healthier fish.