cartman is correct, add up your gph water flow, figure 20x the amount of water you have for a proper circulation, also do you have a cleaner crew?
example:mine is a 75 gallon, about 60 gallons of actual water, after substrate live rock and decor. my total GPh=1235 which is 20.58 times turnover per hour..also how long does your lighting stay on? try to reduce lighting to about 8 hours, unless you have corals or organisms that require more light....check your levels, high nitrates can result in algae, water changes and good tank maintenence is a must.
a cleaner crew if you have a FOWLR would include hermits, turbo snails(they love algae), and some astrea snails will also aid in the removal, as well look into a lawnmower blenny or some actual algae eaters, like tangs, angels etc...