keep in mind that doing pwc
's to maintain low trates and phosphates is completely different from doing them to bring down those levels. If someones tank is sitting at 5 trates and .015 phos, they will have to do minimal water changes to maintain that. now if that same person has 40-60 trates and .5+ phos, they obviously would have to do alot bigger pwcs to not only bring that number down, but to even maintain that same level.
as for what percentage to do and how often. theres absolutely no way to apply a "golden" standard to this. every single tank is different and reacts differently. even if you set up 2 identical tanks next to each other, they are more than likely going to vary in certain degrees. i myself used to do 25-30% bi-weekly, but switched to 15% weekly. and ive had better results. once i get my trates down into the 2-5 range, im going to switch back to bi-weekly