I need some advice, I just bought a used 120 gallon sw tank that came with live rock and fish. I brought it home and cleaned it out and added everything to it. It had been going along just great for about two weeks then I started getting major algae. I went through and cleaned the tank this past Sunday, did a gravel vac and a 20 percent water change(that was all my r/o unit could put out in that amount of time or would have done more) by Tuesday afternoon the live rock and glass is covered with algae. Does anyone have any suggestions on how to cure this and also how to clean the live rock without damaging it. 
You guys had a little tunnel vision on the Algae and missed something. His tank is new and just set up here.
There is cycle information here on this site. Though I have not read it myself I imagine it details on what to expect during a cycle.
Being a tank that large you will have to let it cycle. And with a cycle comes algae. You don’t even need lights on often.. Your photo period being short is probably good so that the algae can bloom and do its job. It will eventually starve itself out by eating up all the nutrients in the water during cycle.
You have a long wait on your hands. Sure there are ways to shave off some time but honestly a tank that large is going to need a bit of time. My guess is at the least a month to be ready for any live stock.
My suggestion is watch the cycle process. Don’t change water for a month. Don’t add live stock. When you see the green algae and brown algae recede (it will eat all its food source eventually) check your water parameters.
Test for Nitrate, Nitrite,Ammonia, Phosphate, Calcium, Alkalinity, and your gravity. The sticky above should have water parameters you want listed … I cant off the top of my head recall what Calc and Alk need to be… seriously.. I just use my lil book. And My marine water chemistry book I have read is now misfiled in my brain. SO make sure to find out what you need those numbers at.
As for Nitrate,Ammonia of course you want those at 0 and holding stable there to be considered cycled.
PS: Currently you will see Nitrite and Ammonia spikes that will be normal. Same with Algae blooms.
FYI There is no sure way to put a timer on your tank cycle time. I just know that a tank that sized will need minimal a month before you can actually add "Some" live stock... Like a fish one week..another week add another... for example. Honestly tanks that size can take months to really reach balance it needs. Not saying yours will but keep that in consideration. The process can not be rushed.
