Reworking An Existing System-Trials And Tribulations
Thanks for the comments and the link.
<My pleasure! Glad I could help!>
I was recently counseled by a pretty reputable local dealer to leave the plenum alone because disturbing it would release all kinds of chemicals I don't want in excess in the system so leave it be.  I don't think that's a good idea. I believe this could be remedied with activated carbon in the sumps.  The other option is shut off one tank at a time, completely drain it,
Rip the plenum out hopefully without damaging the glass and replacing it with course crushed coral and live rock, and add ~ 92 Gal of New Salt water to the system. HELP!!!
<Well, I would be inclined to agree with this guy, actually. My biggest concern is releasing potentially noxious compounds into the water after disassembling the plenum. If you are so inclined, I would consider this a complete "breakdown" of the system, and treat it as such. That means removing everything, executing the massive water changes that you mention, as well as some good mechanical filtration to remove as much of the detritus )which will no doubt be released into the system as a result of this activity) as possible. I think you pretty much have the rest of it wired...don't forget very aggressive protein skimming, too. Also, be sure to monitor ammonia and nitrite for some time after the procedure is completed, and don't add any new fishes or other livestock until the tests confirm all is stable...>
Side note-The first sump is loaded with Aiptasia.  I've been adding peppermint shrimp to the sump but I don't see the shrimp actively eating anything. I could pull the Caulerpa out and try to pull them off or use something to make them release (Fresh water dip???). But I would be taking a significant nutrient reducer away from the system. If the freshwater did would work without killing the Caulerpa that might be a feasible option.
<Well, Caulerpa can leach some chemicals into the system if ripped or otherwise stressed. I'd opt to remove all of the Caulerpa and go with my one of my favorite "purposeful" macroalgae, Chaetomorpha linum. It's every bit as efficient as Caulerpa, but with none of the "dark side" that you need to be aware of when utilizing Caulerpa. I applaud you if you were referring to the Aiptasia when you were talking about removing a "significant nutrient reducer" from your system. These animals are very efficient at processing nutrients. In fact, Anthony Calfo, in his "Book of Coral Propagation", suggests a novel use for excess Aiptasia: use them as a natural "scrubber" in a dedicated raceway, channeling nutrient-laden water into a field of these anemones. Very cutting edge; very cool, if you ask me!>
I also have green and red Cyano bacteria breakout occurring ( High
Phosphates??) This seems to happen when the CO2 stops releasing into the Calc reactor. Right know I have the Calc reactor off for the night cycle. Don't know if this is adding to the problem.
<Well, it is possible that it can be contributing to it...>
I believe there is enough aeration in the sump and enough photosynthesis for me to do continuous run with the calc reactor without dropping the pH significantly.  I don't have a pH or kH probe and I don't know how good my judgment is with the liquid test strips.
<An electronic pH meter is a good investment if you're gonna use a calcium reactor..>
So far I've been watching the tanks for positive response to my adjustments. According to the kits I use I would guess kH to be low and pH to be 8.4 the upper limit to where you want to be. Calcium ???.
<Ask a dozen people, you'll get a dozen answers. My answer is at least 350ppm>
I believe the high iron content of our water to be a contributor to the hazy colors I get from the test kits. I started pretreating all my water with carbon to try to reduce the iron and it appears to be helping.
<RO/DI is a great start to prep source water...>
About our water.  Our water is so hard that if you let it sit with circulation you will have piles of iron and carbonate precipitates on the bottom and sides of the tank. Precipitates so hard they have to be removed with a metallic scraper.
<THAT's hard, ladies and gentlemen!>
If I go a RO unit to deal with this I would anticipate about a quarter of the expected life from the filters in this type of system.
<That's being optimistic!>
What fun.  
<Ain't it, though? Good luck! Scott F.>

New System
Dear Bob,
An addition to the previous message!
<Please copy/paste, forward previous pertinent information, messages...>
Is it a good idea to bypass the Plenum with some of the main tank water, straight into the return chamber?
<The plenum? No... unless it is part of a refugium style sump, and you have another parallel sump to bypass it by...>
I read somewhere that Plenums don't like too much flow through them.
<Think you're referring to something else... Plenums don't have water actually flowing through them. Please read: http://www.wetwebmedia.com/Plenums.htm and the linked files above>
For my return pumps I am using 2 Grundfos 15/50 Selectric central heating pumps with Aquaturn conversion kits on them making them safe for saltwater. I got the pumps free from a mate who is a plumber!
<These are good products... do keep them free of debris... especially the cooling throughput in the front and back internal (volute) areas... not made to pump particulates>
I hope to get it all completed in the next few weeks. I hope you find this letter interesting. When complete I will be more than happy to send you a few photos of the system from sunny England!
Eden Maddocks
<Sounds smashing. Cheers. Bob Fenner>

Re: How long before plenum becomes effective?
Thank you for the reply Anthony.
<very welcome, good sir>
Just so I have things straight. I do have 3" of sand on top of the actual plenum so a total of 4"). Is this what you
are referring to as the DSB? 
<you actually have a 3" Deep Sand Bed (DSB)... the plenum is another matter, and not so critical to denitrification as sand depth and grain size (sugar fine works best for dentrification>
Is that enough? Sounds like you recommend
about 5", but does that 5" include the 1" of plenum area?
<common legend says (and I agree, having used almost tens of thousands of lbs of aragonite sand in my coral greenhouse with and without plenums <no preference> for years>... that denitrification begins to be compromised when the sand bed wanes under 3". Furthermore, aragonite (fortunately!) has a short half life of less than two years. And so, a tank with 6" of sand only has up to two years before the DSB begins to fail. This is partly why so many people criticize DSB technology and fail early with it... getting misadvised and using too thin of a substrate>
I'm just used to people talking about deep sand beds when they aren't using a plenum system. The plenum is 1" thick(1/2" pvc with the 1/2" eggcrate material on top) I put a siphon network in it in case I need to remove water from in the plenum nutrient build-up)
<please don't place too much emphasis on the plenum... it's a fine idea, and your design will work if you have screen on top of the eggcrate to prevent the sand from falling through... but nutrient control issues with DSB, again, have more to do with depth (want thicker) and grain size (want finer. never mix grains...old school). Basically, my advice to you would be to add some more sand in the near future, but don't worry too much about how much or how often. If the sand is dissolving and feeding your system properly, you'll have to add sand in the future anyhow. So, at your pace...simply add sand to always be above 3". I mention 5" to most folks because it is deep enough to buy a reasonable amount of time, but not so deep to be detractive visually. In larger systems, deeper is actually better. but in home tanks the point is somewhat moot>
Thanks. You guys deserve some kind of civil
servant award for all the time you spend answering these questions :)
<thank you very kindly for recognizing, my friend <smile>. Best of luck to you. Anthony>

How long before plenum becomes effective?
Bob,
<Anthony Calfo, in your service>
I looked for an answer, but didn't find one on the site. In your experience, how long does the typical plenum take before it becomes
effective? I have a 38 gallon tank, with a newly installed 10 gallon sump that has a plenum and a small growing crop of Caulerpa. The sand used for the plenum was fresh special grade aragonite. Nitrates hover between 10 and 15ppm, and phosphates between 0.1 and 0.2ppm.
<it is specifically the deep sand bed that becomes "active", and if your sand is deeper than 3" (closer to 5"+) then you can actually see a reduction in nitrates in as little as two weeks (Sprung...and I agree). If your sand is less than three inches...don't even bother to hold your breath <wink>... not going to happen. Need DSB as I suspect you have. Kind regards, Anthony>
Thanks for your help in the past.
Dan

Sand sifting animals and plenum 5/6/05
First, like so many people have said before your site is awesome. I really enjoy reading all the advice and gathering information from other people’s questions. I have a couple of questions. My main question has to do with “sand sifting”. I have a 75 gallon tank with a wet/dry, large Excalibur protein skimmer, roughly 45 lbs live rock, 4 inch sand bed (plenum – however, I forgot to add the mesh between layers), Coralife Power Compact Dual Strip Light with 10,000K and Actinic. 
<The heart of a plenum is the void space under the sand. The screen between the layers functions primarily to protect the plenum. Wet/dries are generally unnecessary with live rock, and often lead to nitrate accumulation. If your nitrates are high, you can remove 25% of the bioballs per week until they are gone and your nitrates should come down.>
System has been set up for almost a year. I had a parasite outbreak that wiped out my tank and it has been sitting with my only survivor, a yellow watchman goby, for 2 months to clear. I also have 2 cleaner shrimp, 10 blue leg crabs, 1 scarlet crab, and 5 Nassarius snails. I am looking for something to keep the sand looking nice – right now have some algae going in areas. I have had in the past orange diamond gobies, which I loved, and they did such a good job but could not keep them from going AWOL. I’m also worried about compatibility with the watchman. 
<Sorry to hear about your parasite troubles. Diamond/sleeper gobies are very difficult to keep alive for long periods of time. Their sifting activity is a search for sand dwelling critters to eat. Even large tanks with very "lively" sand beds rarely have enough of these critters to support these fish. Since many will not accept prepared foods, starvation is common. Also, most of the substrates used in aquaria are very hard on the mouths and gills of these fish.>
I was thinking about a sand-sifting star, but was worried about the plenum. Plus, I have seen mixed advice from you on these. Or should I just up my numbers on the crabs and snails? What would be the proper count with my set up? <Snails and crabs aren't really effective sand cleaners and sand sifting stars are predators on the same critters that diamond gobies are. Sand sifting sea cucumbers are great choices, but require fairly fine sand (1mm and smaller). Increased water movement often helps a lot in preventing algae from growing on sand.>
I am trying to keep a peaceful community tank. I know that I would like to have 2 clowns, a tang (either yellow or Kole), and thinking about a dwarf angel (either flame or coral beauty). What are your thoughts of this combination and could you recommend maybe 1 or 2 others that might fit in nice. Thank you in advance for the assistance. <This sounds like a nice selection of fish. Other good options include many gobies and blennies, royal grammas and peaceful wrasses. Best regards! AdamC.>