Once every few months now , I used to do weekly , but for some reason I’m finding that I get better growth and color when I leave my tank alone for long periods of time
Only as needed. Our main aquaculture system hasn't had a significant water change in ~18 months. Just occasional small detritus siphoning (~5gallons on a 940gallon system) every 4-6 weeks and the little bit that goes out in bags when we ship corals. Some of our service clients get 20% water changes about every 10 days or so. Depends on the tank.