Brown Algae - Phosphate undetectable

[map95003]

My tank is ~7 months old and I've been fighting this brown algae problem for a while now (over a month). It was covering my sandbed and my live rock. For the most part my sandbed seems to be doing much better (with the exception of some small areas), however, this stuff just keep growing on my liverock. I blow it off with a turkey baster and within a few days most of the rocks have it again. I use RO/DI water and my parms test OK..

Ammonia: 0
Nitrate: undetectable
Phosphate: undetectable
pH 8.0-8.1
Alk: 9-10dkh
Temp: 77.5-79.5

I've tested for phosphate with API and SeaChem, both tests show (yellow) 0

I've been running a phosban reactor with PuraPhoslock for a while now, what else can I check that might be causing the problem?

Another observation, coraline is starting to cover the back and side glass, powerheads, return lockline, etc...but it's not taking off on my LR..is this a sign that the rock is releasing something that might be causing the algae?

From my MAG9.5 return pump I'm using a piece flexible black PVC (at least that's what the guy at the LFS told me), could this be leaching something in the tank (if it's not actually PVC) that might be causing the algae problems?

Is this just normal for a 7 month old tank, should I just leave it along as long as my water parms seem OK?

I changed my T5 bulbs about a month ago, not sure if this is the reason. I replaced my 3x10k's + 3xActinics with ATI 3xAquablue Specials + 3xBlue Plus.

For flow I have 2xKoralia #4's and 1xKoralia #3. My mag9.5 return pump also puts out quite a bit of flow...all together this is a lot of flow for a 75g DT.

I know I have a lot of questions but this is frustrating.
Thanks for your help.

[Dingo Dog]

I understand your frustration. From what you're describing, you might need more water flow which reaches actually reaches the problem areas of your sandbed.
I started using the Hydor FLO rotating water deflector (check out Marine Depot or lfs) with 2 of my 1200 maxijets, which allows 360 degree rotation so that the water gently cycles and passes over the sand over and over via the 360 rotation. This allows the detris to be gently stirred up and moved around. In about 3 days, the brown that had almost crusted onto areas of my sandbed was gone. Nothing else was required. So, if everything else seems to be within proper parameters, this might work for you too. I hope this helps solve your problem.