Was the rock cured? If it was cured rock and you filled with all new water, I doubt if the water quality is bad already. Brown diatom blooms are a part of the normal cycle for a new tank. I feed heavily so I am cleaning my glass every two days. I also use lots of snails, grazing fish, and cucumbers to clean with. If it is a brown dust looking algae don't worry. What is a low PH? are we talking in the 7's? If not your probably OK there also. Check you ALK and add some buffer if it is low also. Use only RO/DI water and drip Kalk. Kalk will raise your PH, buffer the system, and grow coralline algae out of control! Keep us informed and if you need any more help let us know. Also as GaryP stated, if you don't already have these other things going try them. If you do your 30% monthly water changes with RO/DI the phosphates should not be a problem, but if you don't do these changes routinely they can creep up on you and are hard to remove once you get a high phosphate reading.