Question re plenums
Dear Bob,
I have had a plenum on a 125 fish and live rock tank for about 6 months.
It has worked exceptionally well, lowering nitrates from over 160 to just a trace above zero. I've always had algae growing on the front of the tank below the sand level, even before the plenum. It's usually red or brown, not a major bloom and easily removed.
Since adding the plenum, it's a little harder to remove (because the space is tighter) and I can't get all of it. In the past few months, it has been getting darker and I now have what I believe is blue green algae in a small line (about 1/4" top to bottom) along the bottom front edge of the tank. I've added powerheads and redirected flow from my main return (about 1200 gph) toward the front, but it's still there. 
Phosphates are ever present in the tank and the front gets about 1.5-2 hours of Southern California sunlight (filtered through dark tinting) every day.
I suspect the combo of sun, phosphates and the deep sand (about3.5 inches) is the culprit, but I don't know how to get rid of it. Any suggestions?
By the way, all the fish (mainly tangs and butterflies) are doing great.
I also just added a refugium and a few crops of Caulerpa in the hope of putting some of the phosphates to good use. Do you think, in time the refugium will help with the blue-green or do I have a bigger problem?
As always, I look forward to any input you can provide. Thanks for your attention.
Dan Pascucci
>>
Hmm, first off, I'm curious whether this is indeed a type (or more than one) of a blue-green (bacteria) algae... you could scrape some off and take a look/see under a medium power microscope (distinctive... no nuclei, organized plasmids, like chloroplasts...), Secondly, even if so, why the sense of urgency to scrub or remove it... some will not/does not hurt (indeed, the stuff is everywhere).... 
A few possibilities if you're adamant though... A bit of tape around the edge where the sun won't shine on the band and where you won't be able to see through (the tape)... Redoing the plenum with a gap (a good inch or two) around the plates and supports (draping the screening down to the bottom) where you can scoot the sand aside and get to the area.
Bob Fenner

Question re plenums
Thanks for the info. That's reassuring. I don't really mind it being there; it's inconspicuous enough. I just have always been under the impression (falsely I guess) that blue-green algae is a bad sign. I guess I'll just keep doing what I'm doing -- removing it when it gets more visible (not a problem lately) and leaving it when it's in small amounts.
Thanks for the help.
You're so right on the impression... and it's falseness... Some blue green is actually a "good sign"... and often, working to eliminate it (at least appearance) results in far more trouble than just ignoring it.
Bob Fenner

Anaerobic/Jaubert/Plenum/Disasters 
Bob, been a long time...I've come a long way since we last talked, don't know if you'll remember me. I'm now a technician at Mote and I do research for Marc Weiss Companies and have a small propagation system...I'm asking as many aquarists as I can...Have you ever seen a closed Jaubert system fail as far as alkalinity, pH, and substrate solidification (i.e....Aragonitic concrete?) At Mote, our 2-year old Jaubert tank is failing, exhibiting coralline bleaching and massive bailout. As a result of such events, our new propagation coral lab will be pure Berlin with surge devices, both air piston-driven and bell-siphon (Carlson). 
Thanks for any observations you can offer
Chris 
>>
Know of several such failures anecdotally... can't say/determine how much set-up, maintenance contributes/ed to cause... 
Bob Fenner
Bob
Thank you for your observations, and your point on maintenance is well taken...Yet the Monaco aquariums' tanks boast such success (guess the filtration systems aren't a big deal when you exchange ten percent of your bulk water a day with NSW!).
<Sometimes only 100% every month... but the point is taken>
I have another theory on the reason why the corals in the tank are failing (although over lighting & crashing alkalinity are obvious and deserving scapegoats)...some of my contemporaries share it, some abhor it; All the corals in the failing system were collected from deep (20'+), offshore reefs (Familiar with Looe Key off Ramrod?) 
<Been there>
Offshore reefs, IMO, should be considered completely different microcosm as inshore reefs, whereas inshore reef are immediate recipients of runoff, and their trophically complex food chain is adapted to consume and keep in check algae (i.e. numerous shoals of tang inshore) and zooxanthellae are more keyed into using ambient water concentrations of N for photosystems then using catabolic ammonia from their host's metabolic processes (which is what occurs on offshore reefs). whereas offshore reefs have the luxury of "infinite" dilution, and having the water "stabilized" by N-rich inshore and abyssal ecosystems.
<Don't know that I agree with this simple generalization... do you have data to back up this speculation?>
I think this has a lot to do with the slow failure of the corals, who were placed in the most unstable ecosystem there is (an aquarium, regardless of size).
These corals (at mote) were collected from a deep environment where 65% of the photonic energy from the sun has already been absorbed and they were immediately placed under 4 400watt 6500K halides! we lost 4 out of 15 corals initially and the rest have gone slowly downhill ever since (however with the recent failure of the system, the regression has increased to a blistering pace). Does any of this make sense? sorry for the lengthy explanation 
thanks again, Chris >>
<Need even more lengthy input to render any real help... Could be many factors at play here... even "just" collection, handling trauma prior to your receiving scleractinians... Be all this as it may/will be... I am concerned with so-called pro-anaerobic approaches as the "Jaubert", "Monaco" style filtration... not reasonably safe and effective... thus I stand on a carte-blanche suggestion to folks who would utilize "plenums" to place them outside the main/display system... as in separate sumps for ease of manipulation... and isolation.
Bob Fenner>

Plenum - to remove or not to remove?
I have a 14 month old 40g Eclipse tank. Learning how to keep this in balance has been every bit as interesting and complicated as medical school, which I finished 4 years ago. I certainly find it easier to maintain a room full of premature babies on ventilators than to keep my tank in balance- the more I learn, the more I realize I don't know!
<A couple of weeks ago, the premature baby comment would have completely freaked me out. Luckily my wife is now at 39 weeks and doing good.>
The burning issue of the moment concerns my "plenum" which is not functioning as such. My tank is overstocked (early ignorance), with 12 goby/blenny type fish (ranging from yellow goby ~1.5cm to flame hawk to 2 alarmingly large Engineer gobies at about 12cm now). I have numerous different polyps (doing well), 50# of healthy live rock, no stony corals. It has a 55w power compact light and a Prizm protein skimmer that produces a couple of oz every 3-4d. I have been battling high nitrates since September. It started acutely with a dead snail and nitrates that leapt from 12.5 to 100 in 1 week, and things have never been quite the same since. In November, I took everything apart and put in a plenum (plastic egg crate, screen, 3" crushed coral topped with 2" aragonite). Initially I did see an effect, with nitrates again in the 5-15 range for a month or so. Then of course my engineer gobies started digging it up. I now have areas where the gravel is 9" deep, covering and killing a powerhead, and areas where the screen is visible. Nitrates have been about 20 for couple of months and just this weekend were 50. I have been changing 10-30% per week using your recommended water change procedures.
My phosphates are also high, the worst was 1.0, now around 0.6. I have been using a Polyfilter since mid-January, and Phosguard since Feb. 1, and have started straining my food as you suggest.
Through all of this, my polyps look good and I don't have any algae problems (hair algae was a problem in Dec., but GARF reef janitors took care of that completely). I don't have terrible ill effects from the nitrates that I can identify. Still, I am sure things would be happier if I could fix this.
It is apparent to me that my plenum is not doing its job in its dug-up state, and in reading it seems I am at risk also for significant hazards related to a malfunctioning plenum. I have been thinking of taking it out, by removing all the tank contents, then vacuuming out the entire substrate, rinsing the bottom, and adding a 1-2" bed of live sand and replacing the tank contents.
Should I remove it? If so, is this plan reasonable? What other recommendations do you have with regards to my nitrate problem? (I plan to upgrade to a 130gal with an Ecosystem-type filter perhaps within the next year, and transplant my current menagerie to that).
Tracy Creek
<Tracy, my recommendation would be to remove the plenum and go with 4+" of fine aragonite sand on the bare bottom of the tank. You are still going to have problems with your fish digging, though. My other recommendations would be to use purified water for water changes, grow some Caulerpa macroalgae in the display, get the larger tank to accommodate all your fish, or find another home for some of your fish. -Steven Pro>

Sand Sifting Star Detrimental to Plenum Setup?
I have 40 gallon with a small sump (10 gallon aquarium) that has a 1" plenum and 3" of aragonite sand. Is it possible that the sand sifting star that is in sump could be removing the beneficial bacteria absorbing/eating it)? Or is it doing more help than bad by stirring it up and removing detritus? Are there better creatures for this? (stirring the substrate that is)
<Good questions... the Archaster star is doing more good than harm. There are other organisms you could use instead, in addition. Please use the search tool, or marine index to read about "Sand Sifters"... on www.WetWebMedia.com
Bob Fenner>
Thanks in advance,
Dan