Cut the time between WC's in half. When you get the situation back under control you can begin to relax one of these changes at a time to find a routine that is easy for you to commit to, if you find otherwise.
How did you determine the Phosban needed replacing every few days? The only way to know would be that you noticed your P04 levels drop, then rise again. Or that you would have observed increased algae but this would incdicate you were waiting way too long to change it out.
If this is true, it shows that you are indeed importing a lot of P04, and shows that the increased WCs (volume and frequency) are needed.
If not, you could have been changining it out too soon, literally throwing money down the drain. Make sure you get your moneys worth as it is expensive.
Timers are VERY important and key in overall tank health. Depending on your livestock, the photoperiod will vary. Considering you have PC's 8-10hrs is probably around where you will need to be, but that needs to be determined as the effect of the change.......with a timer.......is observed over time. 12hrs was a good starting point, but lets make sure its what you "need".
I find this article below informative to visually see the effects water changes have with its included graphs, rather than just from a numerical explanation.
I found that even at 30% monthly I was not getting to the point I needed, so I do 30% every two weeks and found this very effective as not only is the frequency of WC's improtant, but the volume as well each time.......WC's are the reef tank equivalent to the "do over".
http://reefkeeping.com/issues/2005-10/rhf/index.php
A little find tuning is all it appears you need.
