Brown spots on Live rock?

My 75 gal tank has been cycling for about a month I have one damsel, a few saltwater ghost shrimp, and 80 pounds of base rock. I tested my water and found the ammonia to be 0, nitrite 5.0, nitrate 20. I recently noticed brown spots appearing on the rocks and a couple more spots each day. Brown algae??? So i was wondering what this is and is it a good sign? I leave my hood light on for about 8 hours a day, i did use tap water but was told this would be fine for a fowlr, and put a little food in the tank every morning and evening. Also do fish like natural sunlight?

 

Tap water contains lots of silicates! High levels of silicates and phosphates are one of the main causes of brown algae blooms! Check your phosphates! Try to switch to RO/DI water... That will save your time ... BTW , check the algae section of 3reef and read about algae blooms and the possible solutions... I've also started with tap water, however used phosphate binding filter medium (500g) that removes not only phosphates but also silicates... I've used this one: click here.

Good luck!

 

Tap water is a no no in saltwater. You can get a RO/DI unit for around $100 at Air Water & Ice and they are a sponsor here. You will spend a lot more then that on chemicals to get rid of algae problem you are going to have if you keep adding all those minerals in your water.