I have five well established (several years) tanks. I have always had Brown algae and it's a pain having to clean my river rock each week with water changes. My water chemistry is always consistent no ammonia,no nitrites an nitrates of 5-10. My lights are on timers and are on four hours in morning and four hours in afternoon with no lights 10am-2pm. There is no direct sunlight. It seems I've tried everything with exception of testing for phosphate. Any other suggestions?