This is a great idea. I can run something similar off the manifold on my return line, and switch my gfo reactor to a carbon reactor.

Iron certainly does feed cyanobacteria (and pretty much any algae too), that's not a big discovery.

What I don't agree with is the iron being the trigger for the cyanobacteria. My opinion, based on my own experiences as well as hearing/reading talks by leading aquarists/biologists is that it is the imbalance of phosphate between the water column and the substrate that is the trigger. It's nearly impossible to keep cyanobacteria out of a reef tank, so it's the trigger for the explosion that you want to avoid. I've seen cyanobacteria in new tanks where old rock was used, and new saltwater was used. Also causing this imbalance. No GFO.

Cyanobacteria is certainly in the water column when it is on the substrate/rock. Think of the cyanobacteria clinging to the rocks like the fish swarming for the food. Most tanks have enough water movement so the cyano can't form clumps in the water column, but it certainly will if the water is not moving enough. In cases where there is a lot of cyano, I've often seen it floating in clumps in areas of low flow.

Keep in mind there are thousands of species of cyanobacteria.

Thanks. The sump water is also filtered first through a 200 micron sock from the display tank.

I'm not saying you can keep cyano out of a reef tank. But I am saying that the idea of an 'imbalance' as we tend to apply it to aquariums is a biologically meaningless term, and there's never been (that I've ever been able to find at least) any research in any kind of an analogous natural system that would support that idea. If it's an "imbalance", what would constitute a "balanced" state? There's always going to be phosphate in the system somewhere. In older tanks, there's going to be a great deal of it in the rocks from a few different sources. Cyanobacteria doesn't care care whether the phosphate is in the water column or the rocks, if it's there and accessible, and it has everything else it needs, it will grow.

The idea that reducing the total amount of dissolved phosphate in the tank could somehow make phosphate more available to cyanobacteria doesn't make sense. In a tank with no cyano problem, one of three possible states is true:

1. There is no cyano in the tank (virtually impossible if you've ever added a coral, fish, or piece of live rock from another tank or the ocean)
2. There is a cyano predator keeping it in check (if you could identify this predator and breed them, you'd be rich. Or rich-ish)
3. The nutrients needed for cyano to become dominant are not available in sufficient quantity.

Case three is most likely. If you add one chemical intended to lower the level of one nutrient (GFO, biopellets, etc.), and then you see an outbreak of something like cyano, it is far more likely that what's actually happened it that you've not reduced the target nutrient low enough to become limiting to said organism, while adding another nutrient that previously was.

I'm not saying you need GFO for cyano bacteria to grow. What you need is all the nutrients it requires to become dominant present in excess to grow. Old rock in new water might be a perfect example of that case. We only do 10-20% water changes at a time, so without intentional addition, over time the iron levels in a tank will fall, especially if you've got algae of any kind growing. 100% new water has whatever background iron levels were present in the salt mix (potentially quite high depending on the source), and nutrient laden rock that, depending on the rock, has the capacity to raise dissolved phosphate levels by 0.5 to 1 ppm in a couple of days.

I agree there is cyanobacteria in the water column to some degree, but the kind of cyanobacteria that forms mat-like biofilms that are problematic in reef tanks are most likely not the kind that have much or any of a pelagic phase. The mat forming cyanobacteria that we deal with are probably species from the genus Oscillatoria, which form long, oscillating filaments that form dense mats on top of substrates. They reproduce by fragmentation, and unlike dinoflagellates that can let go of the substrate and move in to the water column and swim around, the only amount of time most Oscillatoria species spend in a pelagic 'phase' is when hormongia (lengths of filamentous segments) break off from a parent colony and get blown to somewhere else that they settle to start a new colony.

Pelagic cyanobacteria are from a different group of genera and are mostly non-colonial forming, single celled organisms that wouldn't last long with a protein skimmer and even the smallest amount of mechanical filtration. The exception are species from the genus Nostoc, which can form massive floating mats (perhaps what you've observed in tanks with poor flow), but under adequate flow and decent filtration they'd likely be quickly destroyed and removed. Basically I'm saying that the kind of slimy, substrate coating, mat forming cyanobacteria that we deal with don't pick between being in the water column or being on the rocks depending on conditions. If you're seeing more cyano on your rocks tomorrow than you saw yesterday, it's because the total mass of it in your tank is increasing, not that it's picking a substrate bound state over a pelagic state (otherwise our tanks would look like a red tide).

I picked up Phozdown tonight, but stupid home depot sent me on a city wide run around looking for the half inch PVC fittings I'll need to modify my manifold, so I won't be able to set it up until the weekend.

I also picked up a 100 micron filter sock, and some 50 micron filter fabric to put in it that will live in the filter second filter sock holder in my sump that I've never used to dose the phozdown in to. I'm a little nervous about calibrating it all properly as I don't really trust most phosphate test kits (thus it's hard to trust the "rate" at which phosphate is added to the tank), but I'll start slow and see how I go.

I don't know what section of town you're in, but I find that Rona at Crowfoot has the best pvc fitting selection.
I started out dosing 1/2 the recommended daily amount of Foz Down.
For measuring phosphate I use both the readout from my Hanna tester and the appearance of existing macro algae to tell me if things are going in the right direction.

Which Rona? When I left the 16th Ave Home Depot I called the Bowness Rona because they're the closest but they said they didn't have anything smaller than 2" fittings. The guy at Home depot said they were the only ones that didn't carry the small fittings though, so I drove all the way to freaking royal oak (after their plumbing department wouldn't answer their phone) and they had the exact same selection as 16th ave. I hadn't eaten dinner yet, so I was hangrily fuming. The next closest Home Depot would have been Country Hills, but that's practically driving to Airdrie and they wouldn't answer their phone either.

Rage.

What 1/2 inch fittings are you after?

For plumbing fittings I have flat out stopped going to Home Depot, very little selection and horribly organized. Hit the Rona's. Guy on the phone I'd an idiot. Have 2-3 time the selection of Home Depot and way more organized. I use the one on McLeod and southland but the crowfoot one is good as well. I live south so it's easier.

Before you start your LC dosing (which I have not done, so only passing on what I read), I know that Reefsupplycanada used to sell 10 micron socks to use with Fozdown. I would hate to see your tank suffer another mysterious set back that may be ultimately traced to LC flocs that escape your 50 micron net.