Help... Cyano!

My 125g mixed reef had been running with no issues at all for over a year.  Then about 2 months ago I started to get some patches of cyanobacteria/red slime.  I've since the tank cycled I've never had ammonia, nitrites, or nitrates detected.  I still wasn't seeing anything, and it hit me that I hadn't changed my lights (2 x 400 watt MHs) or the filters in my RO/DI (Reefkeepers Extreme Typhoon III from Air, Water, & Ice) for a year.  So I thought I'd get new bulbs and replace the filters and everything would be fixed, however, the cyano continued to advance.  Next step was phosphates... I was using the red sea drops and I don't know if it's just me, but it is near impossible to tell the difference between 0, .10, and .20.  But I think I was around 0.10... but I decided to get a digital PO4 checker and run GFO to bring it down.  It has been around .04 ppm for several weeks and the cyano is continuing to advance.  I started to do more frequent water changes (even though I'm not detecting any nutrients I figured this is would be a reasonable idea).  One thing I had noticed was my pH was slowly drifting downwards, it typically hung around 8.10 to 8.20 depending on the time of day.  However, it had come down to about 7.80 to 7.90.  I run a Ca Rx so my Ca was at 480 and Alk was 10 dkh, which is very stable for me.  So I started testing more parameters to see if something like Mg, O2, O3, or Iodine was possibly off.  Mg was the only thing that was low (around 1,140 ppm).  I've been raising Mg by 18 ppm per day via Kent's Tech M (which is liquid Magnesium Sulfide and Magnesium Chloride) mixed with my top off water.  The directions say not to raise by more 25 ppm per day (RHF has said limit changes to 100 ppm per day), but I figured I'd play it safe with 18 ppm.  Anyways, I'm now back up to 1,340 ppm which is where I'd like to stay (that's the recommended level for my 1.025 S.G.). I also changed my reactor media to maintain Ca, Alk, Mg.  But now that my Mg is back up, my pH came back up with it, around 8.05 to 8.15 now.  Good news right?  Well, the cyano is getting worse, the colors on my corals look horid, a large colt coral has bleached and I've lost one good sized acro colony and a deresa clam.  I've also lost a firefish, a starry night blenny, and one of my two cleaner shrimp.  I've never had a fish die (if it's survived the first week) in my 125g before.  Is it possible that the pH coming back up over a week and a half caused this, or would a change of 18 ppm in Mg a day be capable of this?  And what should my next steps be in this cyano battle?!
Tank info:
125g (46L x 22.5W x 28.5H), pretty small sump due to the 46" stand, with a 15g frag tank plumbed together
2 x 400w 14,000K MH
The protein skimmer is a Marineland Marine Pro 300 (was somewhat pushed in this direction since I'm very limited on space in the sump and this design has the pump sitting under the skimmer instead of next to it)
Chaeto in sump
Circulation: Closed loop with QuietOne 9000 (2328 gph), return pump is a QuietOne 4000HH (pump is 980 gph with no head, however it goes through a chiller), and 4 x Koralia 2s connected to wavemaker (260-680 gph each = 1880 gph during the day, 1040 gph at night).
Temp stays between 78.4-79.0 (running a chiller)
Salinity is stable at 1.025
NH3, NO2, NO3: 0
PO4: .04
pH: Around 8.10
Mg: 1340
Alk: 10 dkH
Ca: 480
(I'm currently at work so I don't have my most recent numbers if details on O2, O3, etc are needed, but they were good)
Replace and GFO and GAC every 1-2 months
Fish: Blue Tang, Flame Angel, 2 Osc Clowns, Yellow Watchman Goby, Firefish (just 1 now), Royal Gramma
Corals: Several acros, birdsnest, a couple echinos and montis, colt coral, white finger coral, toadstool, pulsing xenia, hammer, frogspawn, ricordia (I'm sure I'm forgetting some)
Have been doing 30g changes every 3 weeks for the last few months.  Had previously been doing 30g changes every 2 months.  The increased change frequency has not appeared to be helping.
Sorry for such a lengthy post, it just seems like most of these type of threads end up starting with no info and just a problem and then some details slowly come out over the next few days.  Let me know what other info is needed for some help!
Thanks!
Eric

 

The bulbs were replaced a few months ago when the cyano just started. It wasn't until the Mg and pH began to come up within the last 2 weeks that the corals started to look like this.

Sure, the 4 k2s are only 1880 gph (15x turnover) by themselves... but the closed loop has several returns through the rock work and has very little head pressure with a 2328 gph pump. I would have thought that even if I'm only getting another 1880 gph out of the closed loop that 30x turnover with flow in many different directions would be enough to prevent dead spots (that's still excluding the return pump). Is it worth upgrading my PHs to K4s? Or perhaps put in 2 more PHs that are on during the day and off at night (just use a timer instead of another $342 Koralia Wavemaker).

Good point on the kits.. my Red Sea kit is about a year and a half old now (which is what I've been using lately). I do have dip sticks that are newer that I haven't been using as of late since they aren't as accurate and I've been looking for any possible sign of NH3, NO2, NO3.

 

Okay, I'll try sticking another pair of powerheads in there to try and hit different areas. Has anyone used the modded maxijets? They are supposed to do 1600 gph. Trying to decide between that or 2 Koralias (either the 1050 Evolution or Magnum 5).

The phosphate checker is giving me a .04 reading.. I can't imagine that's high enough to cause this cyano bloom. Just used strips to test for Ammonia, Nitritres, and Nitrates and they all showed 0 - same as my drops but my drops are over a year old.

I have reduced how much I feed over the last 4 months as I've been dealing with this issue. I was originally feeding frozen food once a day, alternating betweeen Rod's Food and Prime Reef (one cube). And I was feeding 15 mL of phyto and zooplankton 3 times a week. I am now feeding every other day (no fish looks skinny yet) and doing the phyto/zoo once a week. I will occassionally rotate rotifers or cyclops in for the phyto/zoo.

 

Wow, the cyano I'm dealing with is not even close to what you had at China Garden. I'm beginning to wonder if I should give a more serious skimmer a try. The Marineland Pro 300 does a good job of pulling brown smelly gunk, but I'd like to see what I could pull with something like H&S's A150. Since I have a very limited amount of space in my sump, I think I could pull off an external one like this:
http://www.aquariumspecialty.com/catalog/product_info.php?cPath=34_184_94&products_id=450

Again, I'm not seeing any Ammonia, Nitrites, or Nitrates and have been keeping PO4 pretty low (around .04), but like mthomp mentioned, the cyano could be what's consuming excess nutrients.

From what I've been reading on RC, H&S skimmers seem to be very highly regarded. Does anyone have any experience with them?