sand turning brown?

this tank has been setup for about a month and a half. salt has been in and stable for a month. I have about 35-40 pounds of live rock in there with no fish yet. my question is...why is my sand starting to get patches of brown? I swear it was fine yesterday. My pH is low.....7.8 or so. As for the other water parameters....i haven't checked yet. I just got a full kit today and wont be home again til tomorrow. The lights are on 5 hours a day for now, but they are just standard flourescent bulbs I've also done a water change of 5 gallons for the last 3 weeks. Do these brown spots mean my tank is cycling, and will they clear up?

i have three koralias. a #1 and a couple #2s in addition to an emperor 400 and a canister. (75g tank) I have the emperor on the left with a powerhead up top shooting right. the other 2 powerheads and the return from the canister are on the right. those two powerheads are blowing up to the left and down to the left. Does that make any sense?? I kinda just confused myself. The brown in the sand doesn't seem to favor and specific area of tank.

The Emperor also has an airstone in it to provide more oxygen....is this a waste of time?

none are really pointed at the sand.....it tends to make dunes! haha. maybe I'll change them around to see if anything changes. This is my fish marine tank, so i hope this is a sign of cycling!

Score! haha