My tanks are all way over filtered, so I do 10% every other day. Works well for me.I've been thinking about the effectiveness of water change size vs frequency. My understanding is that generally less then 20% per change is a waste of time.
Does tank size play a factor in this? 20 percent of 300 gal is far different than 20 percent of a 55. My largest tank isn't a concern but I am still experimenting with my 38 gallon tanks. Each tank has a single large bp...they muck up the filter pads every 2 days. And I'm talking mud like. All the bio sponges I leave be, and the pothos helps. Ideally if I had space I'd put them each in a 55 but the fish room is a weird, wedge shaped space with only one long wall (the 225 is on that side).
Unless there's an obvious problem I rarely test with API kit. Mostly I do the eye test and monitor. The parrots immediately color up intensely and 'perk up' with fresh water, so I look for these signs. Increased activity (zipping around, fins out and up) are my indicators. So right now I'm comparing back to back smaller w/c (20%) versus one small (15-20%) and one large (50%) per week.
W/C schedule for the 225 remains the same, twice a week at 50-65% percent. No issues and lightly stocked (11" Oscar, 6+" BP, 6+" Severum, 2 syno catfish (5" and 7" inches). Because most if not all of these fish are still growing I maintain the same regimen. I believe the Oscar's ultimate size may determine if more w/c is needed. Since he's only around a year old I expect him to make even more waste down the road. If the BP gets to 8+ inches the game definitely changes.