Drewman!
As far as selecting a RO unit, I think it is the only way to go (unless you have access to free distilled water...) My personal preference is to get a unit around 80 g.p.d., as you can make your water fairly quickly for a large number of tanks (I use a roughneck trashcan on wheels with a lid and make a few days worth of water at one time, on Sundays I can do the water changes by taking Saturday's water and making enough fresh seawater for the changes Sunday (I let it stabilize overnight, aerated and stirred for 24 hrs)
As far as the Cyanobacter goes, I concur with the statements about this being only one of the factors that influence blooms. Aquaria that have problems with cyanobacter species usually have a combination of not only chemical/nutrient problems, but also physical and biological problems varying from lack of competition for nutrients to too long a photoperiod. Usually all the factors that influence cyanobacter spp. growth is present, but some increase in one factor may be the key to producing a bloom.
The chemical factors for cyanobacter can have many sources, but usually center around the availability of nitrates or a nitrogen source (could be ammonia as easily as nitrite or nitrate, but usually nitrate and/or DOM in stable systems) and phosphates. At this point you are addressing one of the main sources of phosphate, in your top off water, but other sources of phosphate are readily available in your inputs to the tank: feeding (especially flake), carbon from coconut shells, and occasionally the salts for synthetic seawater mixes, although that practice has pretty much fallen by the wayside now. Many municipalities use phosphate additives to reduce the possibility of lead in drinking water. Older systems use polyphosphate resins to soften the water in areas where the raw water has large amounts of calcium present. It would be very important that you use some type of water purifier to remove these sources of phosphate. Please note that although many of the RO units have a rejection factor of 99% that often does not mean that it is removing 99% of the phosphate! Phosphate is a small enough molecule that the RO membrane cannot efficiently remove much of the substance. In many cases, you will find that the companies (check spectrapure's web site for an example) cannot guarantee that they will be able to reduce the phosphate level at all unless the water is further processed by a deionization resin column (i.e., RO/DI water), and even then, the level may be reduced by only 50 to 70% based on the input-water concentration of this ion. I would try to limit the input of phosphates, but don't be so concerned with this that you feel the need to stop feeding your fish, rather, find a way to export the phosphate. There are many ways to do this, my personal preference is to use a refugium and grow macroalgae. This removes the phosphate as you harvest (and discard) the calerpa/halamida/whatever you grow to take up the phosphate.
Algal turf scrubbers do the same, although I am not a big fan of these systems. The use of a good skimmer is another way of removing these compounds, and it will reduce the levels of Dissolved Organic Materials (DOM), but skimmers will remove any phytoplankton and other particulate materials that are food sources for many of the tanks inhabitants as well. On this you need to make a decision on whether you want to develop competition for the resources or eliminate some of the overall problem by skimming. I personally have pushed my skimming to running only if the cyano gets out of hand, and I have not been skimming for almost 4 months now with little or no cyano in the display tank (DSB, 2 years old, 6" no plenum)
If you are testing for nutrients and show little or none in the water column, keep in mind that if you have an established cyano growth, as soon as you are adding nutrients to the system, the cyano will be taking them up. Another means of removing these nutrients would be to siphon up the cyano and discard it as part of your water changes. This not only removes the organism itself, but exports the nutrients that the organism has locked up as well.
As far as the aluminum oxide compounds go (phosphate sponges), I don't like them, although I have used them in the past. They have their own impact on the water column, and
sarcophyton spp. will usually let you know that they don't like them by either shriveling up and shedding a coat of mucous or dying, not what I would consider a good exchange for getting rid of the cyanobacteria.
There are physical parameters that affect the growth of Cyanobacter as well, long photoperiods and water circulation. Good water circulation inhibits the growth of cyanobacteria mats, and for mats to develop, there needs to be large amounts of light. These organisms are neither plant nor animal, but have characteristics of both. They contain a red chlorophyll-like substance that allows them to photosynthesize carbohydrates in the presence of CO2 and light, but they also have the ability to capture substances for growth through a diffusion process (active transport for this organism is fairly slow, but does play a role in its growth). Slow water movement increases the likelihood that the organism will be able to capture foods, as well as the likelihood that the concentration of nutrients will be higher in still water (diffusion, remember?). With this in mind, cyano should be easily outcompeted for limited resources in a well-diversified system.
Biological competition in a well established DSB system will control most of the cyano problems listed above, as well as the removal of cyanobacteria itself, as there are many organisms that live in a diverse sandbed infauna system that consume cyanobacter spp. as part of their diet. Rob Toonen and Ron Shimek both have recent articles about this subject, as well as Sam Gamble's articles on live sand beds. Peter Wilkens spoke on the subject at MACNA this year, and suggests that one way of solving cyano problems is to find someone who's tanks do NOT have cyano problems, and get some of their sand to introduce cyano-consumers to problematic systems.
As far as ornamental fish consumers of cyanobacter spp., the stripped Bristle-toothed tang (Ctenochaetus striatus, see pp734-35 in Baensch Marine Atlas). Under the feeding aspects, Baensch lists "...C. striatus sucks the thin covering of diatoms AND poisonous blue-green algae (cyanobacteria) from the bottom, making it one of the few herbivorous spp. that can be poisonous (ciguatera)..." . As the fish keeping is not my forte, I will leave any suggestions for other cyano-consumers to Doug or Dick or Alice (Horge? Martyn? care to chip in?) Keep in mind that problems with cyanobacter blooms are usually multifaceted, and having the cyano consumed by a top-of-the-chain organism generally does not remove the root causes of the bloom from the microcosm. You need to consider putting in place some method of export of the nutrients and a limit to the import of them to deal with the problem more thoroughly.
Hope this helps, sorry for the long post.
Here is a link for food web based tank management:
http://mcnet.marietta.edu/~mcshaffd/...ChainsandWebs4
------------------
Tom <"{{{{>(
(TDWyatt)
Wise men talk because they have something to say; fools, because they have to say something. -Plato
) So I didn't think I would need one. But since my tank is about 8 months old and I still have cyanobacteria really badly, I believe cyanobacteria and not using RO water go hand in hand (am I right?) and am planning to buy a unit shortly. My question is, can I get a good unit for under $150? It doesnt have to make much water, Im only looking at around 20gpd. As for the cyanobacteria, I am not 100% sure that that is what I have. It is reddish brown color, and it covers about 50% of my sand. Does this sound like it to you guys? Is there anything such as crabs or other critters that will eat it? Thanks a lot. Oh and, am I the only person that has crap like this? Because whenever I see a pic of somebodies tank it looks SO clean I get upset when I look at mine.
I've been there, just ask Alice, Doug or Dick how I cried on their shoulders. Since all I had in the tank at the time was hermits and snails, (i.e. NO corals, NO fishies) I turned off my lights and didn't feed for a week! Hermits and snails dug around for delicasies. Success!!! I've been cyano-free since the latter part of August 
