5-10gal every two weeks isnt really enough, IMO. I do 25-50% water changes (depending on tank size) every week, and my zoas grow like crazy. I think the term 'dirty water' is overused, and misunderstood. I'm not trying to be harsh, but from my experience, good water parameters are better than disolved 'food' for the zoas. Doing water changes replenishes trace elements that get used up in the tank. If you can, bump the water changes to 10-15gal weekly.
By 'depending on tank size' what I mean is in my 20gal tank, I change nearly 75% of the water bi-weekly. With the rock and propagation stuff in there 75% is about 8-10gal.
In our 40gal tank, I do 10-20gal water changes bi-weekly. But when our 150 is setup, it will likely be 20gal water changes, twice a week.