Depends on the bioload in a tank and what fish I have in there, but I've found with most fish that more and more often is better. Sure, you can get away with 20% weekly changes with many tanks, but in most cases the tank looks better and the fish seem healthier with more than that. I had an african cichlid tank that I changed 50% of the water twice a week and did a big 80% change on monthly. Never crashed the tank and those fish were really healthy. I couldn't get them to stop breeding lol.