Hmmm... This is a topic that continually comes up.
Phosphate removers work for some.
Changing bulbs works for some.
Water moverment changes work for some.
The most likely source of cyanobacteria control is nurtient limitation. Given that, ask yourself the question, which nutrient is the easiest or most effective and most practical to control?
It is my understanding that Nitrogen and phosphate are major nutrient requirements for marine algaes. This appears to extend to cyanobacteria. There are other nutrients required, but in vastly smaller quantities.
There have been open ocean experiments done to determine the absolute concentrations at which phosphate and nitrate limit microalgae growth. Those concentrations are a factor of at least 10 lower than we can measure on hobby test kits. You can get zero readings on your test kits and still not be nutrient limited for single celled algaes. I think this principle extends to cyanobacteria as it is a single celled photosynthetic organism.
As mentioned earlier more nitrogen is required than phoaphate; approximately 16 times more (redfield ratio). Yes, cyanobacteria can use other sources of nitrogen than NH3 and NH4, but it still needs 16 molecules of nitrogen for every molecule of phosphate.
So is it easier to reduce phosphate to less than one molecule for every 15 molecules of nitrogen or to reduce nitrogen to less than 15 molecules for every one molecule of phosphate?
How do you reduce a nutient in your tank? There are 4 choices that I know of: dilution (water changes), adsorbtion (phosphate sponges, protien skimmers...), absobtion (uptake by another organism), reduction.
Given the low levels of nitrates and phosphates needed by cyanobacteria, and the levels at which they are continually added to our tanks by feeding, dilution is not a practical solution.
I know of no chemical that adsorbs nitrogen in its various forms and phosphate adsorbtion does not work for everyone. I suspect that this is because so little phosphate is required by sincle cell photosynthetic organisms relative to rates of addition and what can be adsorbed.
Reduction is what happens in our live rock and sand beds when bacteria break the bonds in molecules to use one molecule and release the others as a byproduct. For sandbeds the released molecule is nitrogen (hmm... cyano can fix nitrogen...). This is one of the main reasons given for using live rock and sand beds in our tanks.
Sand beds and live rock also support all manner of organisms that consume detritus and lock up nutrients in the form of living flesh. I will leave this can of worms at this point.
That leaves us with what I think is the best form of absorbtion, other algae: macro algae. Macro algae, being much larger than single celled organisms, suck up a LOT more nutrients than said single celled organisms (like cyano bacteria). According to Plantbrain, when growing properly, they also suck them up a lot faster (this is improtant!). This is the reason a lot of folks are adding refugiums. They are a great source of nutrient absobtion and export.
This is also what has worked for me to control cyano in my tank. As long as my macros are growing well, there is very little cyano in my tank. I do not pretend to understand the entire story here, and I encourage everyone to find and read Plantbrains posts in the Marine Plants forum. The short story seems to be that if your macros are growing well, they outcompete (nutrient limit) the cyano. The limiting nutrient is nitrogen in the form of NH3 and NH4.
This isn't the end of the absorbtion story though. What a lot of folks don't realize is that macros also need a lot more available nitrogen (and phophate) to keep growing. Your macro algae can become nutrient limited as well. Once it stops growing well, nutrients again become available for cyano growth. Plantbrains solution?
Dose nitrates (and possibly phosphates). :eek1: :eek1: Yep, dose nitrates to keep your macros growing well and they will nutrient limit the cyano.
I have done this repeatably in my tank. Rather than dosing continually as plantbrain does, I wait for the cyano to start to bloom as a sign that the macros are nutrient limited and then I dose calcium nitrate to kick start the macros. I have done this 5 or 6 times now (every couple of months??) and each time, the cyano mostly dissapears within 3-4 days.
I say mostly because I have never completely eliminated the cyano. it shows up as small bits here and there, but nothing very noticable. I have no heavy mats growing anywhere.
Note: I have also needed to dose phosphates on one occasion to kick start my macros.
I should mention that since I keep seahorses, I have lots of macros in my main tank as well but this is not an issue for me. For those who want to keep an algae free reef, you would need good herbivours to manage macros in the main tank.
OK, I didn't intend to get so long winded, but what we really have here is a toolkit for managing cyanobacteria. Pick and choose what works for you. If one thing does not work, pick up another tool and try it.
Fred
