water flow

i have a 55 gal w/ 500 gph on my return. how many powerheads and of what size and positioning in the tank should i be using?:-[

 

First, what do you plan on keeping
This will determine how much flow your gonna want.
As far as positioning, you what keep dead spots to a min and not directly blast any of your corals. Every tank is different so it's really something you have to play with

 

Hi and welcome to 3reef, first of all!
As for water flow, their are many variables that need to be addressed before making any type of decision as to whether or not you have enough flow. Corals, amount of and placement of live rock, size of powerheads, and number of them as well.
I can tell you that by positioning the powerheads so that the water stream banks off of the glass enables the flow to be dispursed over a greater volume of water as well as reduces laminar narrow streams of water that often times is damaging to corals!

 

It takes some experimenting with positioning your powerheads to get a flow you are happy with. I use two Koralia 3s. One is in the back bottom of the tank pushing water behind my LR wall to keep deitrius from settling on the bottom. My other one is high in the front right side. I seemed to get the best random looking flow in this position. Unfortunately, the PH is right in front.