It's not how much flow you have, but flow patterns.
flow should gently break the top of the water, gently blow over the corals and through the rocks - and be able to kick up some detritus off the sandbed. Sometimes this is accomplished with one powerhead, sometimes six. It just depends... and it takes time and experience to know what is best. I can't really advise you on it unless I really look at the tank and experiment with it.
You should always maintain your sandbed, either biologically or mechanically... You should also have methods and equipment in place to take care of built up dissolved organics and particulate organic matter. The only reason I mention this is because flow rates is not necessarily what keeps a tank clean. water flow helps carry O2 and CO2 into and out of the tank, as well as other dissolved gasses - which regulate pH. (There are other things that do this, but gas exchange is a major reason for low pH.) Flow also helps carry food to corals, and waste away from corals. Another reason to have some flow is to keep detritus from building up to dangerous levels in dead spots in the tank (only in spots that aren't maintained.). There is a great importance in flow and flow rates, however, there is no magical formula that will create a healthy tank - there is only experience in knowing what to look for and the reasons/concept behind WHY you have to have a powerhead or two in the first place.
