I'm not an expert at this by any means and I haven't been in it a long time but from all the advice that I have gotten and from my experience this is what I have noticed regarding water changes. I currently have a 90 gal. tank with approx. a 15 gal. sump and I do a 10-15 gal. water change weekly. Not just because someone told me to do it. But because of the benefits of my tank. I test my water weekly, say on Fridays and do my water changes on Sunday. I seem to have a stable environment and I think it is because of my routine. I think my corals are use to this. If you are watching your tank all the time, not only will you know that you need to do a water change because of say what your levels are showing you, but you will know by the way that your corals are looking. Even though things seem to be stable, my corals always look like I have given them a boost every time I do a water change. Even though things are not bad, they like it! I adjust the amount of water depending on the levels. Say I might do an extra 5 gal. water change if my phosphates are rising. I think that keeping things constant works great for me. Since every tank is different, such as bio load, etc., your tank will talk to you and tell you what it needs. Personally I think that doing a large water change less frequently would be like shocking your tank. Also I think that when it comes to the sake of time, making your water prior to the time that you do your water changes sure saves a lot of time. And then when I'm ready to do my water change it is quick and easy. Maybe not great advice, but hope this helps!