sounds like cyano to me to very hard to get rid of
many things can cause it
old lighting
over feeding
high nitrates
poor circulation
excess nutrients and undisolved organice
heres a small article on it
Systems that have problematic amounts of cyanobacteria almost always have some combiation of physical, chemical and biological problems. Often there is not a unique factor involved, but rather a sum of contributing effects.
Physical
Water flow and illumination. You mentioned that the cyano are growing mainly in areas of high flow, so low flow seems not to be the problem here
Chemical
This is a long category, so there will be several paragraphs here.
Often there is some problem with either nutrient input or export. Jaubert systems are suseceptible to accumulation of phosphate. They deal with inorganic nitrogen pretty well, but unfortunately, phosphate doesn't have any forms that are accessible biochemically that are volatile. So you won't be getting rid of phosphate via a pathway analogous to denitrification
So you need to have some sort of nutrient export pathway in the system. That can be a skimmer, or it can be an algal scrubber. Some people use alumina resins to bind phosphate. I usually don't suggest those, since they are not without impact on the organisms in the system, and they have an impact on other water chemistry parameters as well.
You mentioned that you have done some testing for nutrients and that has shown nothing. The testing that you should be concentrating on is testing of the input water into the system. If you have a good growth of cyanobacteria in the system, it is likely that they will be snapping up inorganic nutrients about as fast as you are adding them. Moreover, if you had one nutrient that was high and the other low, you would not know if the nutrient that was high in the system was really "the problem" since the aglae might actually be more limited by the nutrient present at low concentrations. Test your input water, or get a water assay from your friendly local water utility. Things to look for are additions of phosphate for lead conrol, etc. Although the use of polyphosphate water softeners is low these days, that is another possible issue, and something that your test kits might not detect.
That handles most of the input.output scenario aside from feeding. Some people suggest limiting feeding in cases like this, and I always have a skeptical reaction to that. Sure, if you are grossly overfeeding the system, you can cut back, but I think most often this sort of advice results in starving fish and other organsisms. Some people are very fond of the model of an aquarium as an "ecosystem" and believe that it should produce adequate food. Well, often that is not the case. I'm more of a "cross flow of nutrients" theorist, meaning that I feed my tank more than most people would, but I have a solid nutrient export mechanism working. A sand bed is not a nutrient export mechanism, except perhaps for nitrogen. Sand beds are mainly nutrient processing areas. Nutrients are rearranged, sometimes into very desirable forms, like invertebrate larvae, but nutrients other than N don't leave the system due to sand bed processes.
So I strongly suggest that you get some form of nutrient export going. Again, my preferences there are either a skimmer or an ATS unit. Alumina is a possible special-purpose phosphate export mechanism that is not without other impacts
Biological
Here is an interesting question for you. Cyanobacteria are often pretty good food organisms, yet you have large amounts of it in your system and it isn't being consumed? Why? Often it is because you don't have appropriate herbivores in the system. A number of organsisms that happily much on cyanobacteria live in sand beds. So a good question is whether or not it is possible that the sand infauna in your system either never was seeded properly in the beginning, or for some reason may have been degraded over time. People have literally written books about live sand, and there are a number of good articles on this subject on the internet. I would suggest reading mainly the contributions by Rob Tonnen and Ron Shimek on this subject. Rob also wrote a two-part feature in FAMA on this subject not too long ago, and it was very worthwhile reading even though there were a couple of minor bugs in the chemistry discussion.
A piece of advice from one of the oldest hands in the reef aquarium world, Peter Wilkins, is worth repeating at this point. Sometimes a cyanobacteria problem can be resolved by seeding the tank with some sand from a system that doesn't have a problem with cyanobacteria. You also might want to add a small quanity of fresh live sand to your system.
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It is possible that you might be able to buy an "ornimental" organism to help with the situation. The comb-toothed tangs often feed on soft algae. For example, the Baench Atlas says this about _Ctenochaetus striatus_ "_C. striatus_ sucks the thin covering of diatoms and poisonous ble-green algae from the bottom, making it one of the few herbiverous fish that can be poisonous (ciguatera.)" These fish are actually rather attractive when small, and you might consider obtaining one. I typically don't give animal husbandry advice. In this case, I'll turn over the discussion to the group. Any fish in this genus may potentially help with your problem.
However, keep in mind my advice that cyanobacterial issues usually have multiple roots. While a given organism might eat cyanobacteria, eating it doesn't remove those nutrients from the system. You still really need some form of nutrient export mechanism to deal with this problem in totality.
