When you dosed carbon before, did it give you the numbers you are looking for now?
Also, in the other thread, you were thinking to add a little power head for the cyano, right? Did that happen?
I think it would be easier to return to a routine that you are comfortable with and have had success with, than make a new project. Unless you're into like tweaking your system, that's fun too but it kinda sounds like you just want the nitrates number down for a little while to see if it clears up a bit of algae that's hitchiked in, not a new commitment. Plus you'll be able to back off easier if it works. Like maybe 30ppm is fine for your tank except this caulerpa and cyano got a foothold so they need to be knocked back, but you don't need to make a big change to your system to do it.
I'd try dosing first $.02.
Its a 20g long tank the returns run into the skimmer area that has an 8" baffle to keep the water steady for skimmer the. There is and actually i think its a 4" baffle to hold a 3" sandbed and keep the sand from rolling over into the area with the return pump and phosban reactors
I have been blasting my rocks 2-3 times every evening when I get home from work. For what its worth PO4 was at 21ppb a few weeks ago so at least something is coming down. As far as flow I have 4 power heads in a 55g. Two on each end. I pulled a lot of the Grape Caulerpa out and still pulling some I added Cheato hoping it will out compete the GC. I did a lot of research on NOPOX a few years ago spoke to Aviad on the phone a few times and was trying it after no success I finally ripped all rock and my DSB out and started over with new rock and shallow sand bed
IMO, you have a few options here. But first, I'll rule out the options that aren't going to work. Water changes and more rock. Water changes aren't going to permanently reduce nitrates. It's an expensive bandaid that's will last a day or so, as you're already experiencing. As Rocket pointed out, more rock will not reduce nitrates. Whatever bacteria your tank needs is already there and processing the amount of waste available. More rock is not going to do a thing for you.
Now, the options that will work. The simplest, maintenance free method will be a remote deep sand bed. Be creative, you may be able to make it fit. For example, a stand/shelf above the sump to hold a container of sand. It only needs to be about 6" deep. If you can make it work that will be your best option as you will now be truly exporting nitrates into nitrogen gas. If you have a refugium, convert it to a rdsb, but filter the water prior to feeding the rdsb with a filter bag.
If that option is impossible, an algae turf scrubber is another. Of course if you don't have room for a rdsb, you probably don't have room for an ats. If you can fit it, you have to understand that it only assimilates nutrients and will require that you export them through harvesting, a weekly chore.
Finally, there is carbon dosing. It sounds like you already know about it so I don't have much to add other than another possible route using bio pellets. Every tank is different and while vodka, etc. works well on some tank, other options work better on other tanks.
Completely disagree on the rock point. Sufficient quantities of high quality rock can and do act very similar to a DSB as they harbor anaerobic chambers deep within that house denitrifying bacteria. What is the basis for your claim that more rock will not work?
How old is the new setup? As in when did you redesign the tank with a shallow sand bed?
http://www.advancedaquarist.com/2012/4/invertsCyanobacteria are capable of diazotrophic growth, which means they are able to convert or fix dinitrogen gas (N2) into ammonia (NH3) using the enzyme nitrogenase (Postgate 1998). Ammonia, in turn, is further assimilated as the amino acid glutamate (glutamic acid), an example of biosynthesis. Glutamate can be converted to other amino acids and proteins, a so-called metabolic pathway which is important for organismal growth. Nitrogen fixation is hampered, however, by the presence of oxygen. Cyanobacteria have solved this problem by making use of heterocysts, specialized bacterial cells that are protected from the photosynthetic oxygen produced by the bulk of the cyanobacteria in the mat by multiple cell walls (Fay 1992). Heterocysts fix and translocate nitrogen as ammonia to the photosynthesizing cells of the mat, whereas these latter cells provide the heterocysts with organic carbon. In this way, the consortium of cyanobacterial cells is able to convert carbon dioxide (CO2) and dinitrogen gas (N2) into organic compounds for growth. This strategy allows cyanobacteria to overcome nitrogen-limitation, enabling them to grow in an oligotrophic environment with low levels of nitrogen. Examples are Anabaena sphaerica and Nostoc punctiforme.
I was looking for something else, and I came across the cyano + nitrogen thing I mentioned before. Thought I'd leave it here in case anyone was curious about it, pretty neat stuff.
http://www.advancedaquarist.com/2012/4/inverts
Good luck with the vinegar Kissman!