I have modified plenums in both of my 20g. reef tanks. What I did was use penn-plax UGF's which I capped off. I drilled out one of the caps and inserted some black 1/4" hose more commonly used for ornamental pond pumps. There's about three feet of the black hose snaked under each UGF plate, with small drill holes about 2" apart drilled into the hose itself. About 2" of hose sticks up through the cap above the UGF plate, which i've capped off with a black plastic cap and tucked into the sandbed along the back side of the tank.
This "underhose" creates something a little new to the hobby, called a super plenum. The big controversey over plenums is that they may (keyword: may) become fouled over time, or if the sandbed is too deep, could potentially turn anoxic. With the hose modification, I can actually draw off water from underneath the plenum plate in the future if i'm worried about compound build up. By slowly drawing off any scumwater from under the plate, I can slowly drain out the water with an artists syringe, or even a modified powerhead, any time I think things have gone wrong in the plenum. It's like having a plenum with an insurance policy.
However, the jury is still out on plenums and DSB's. Just as many people have them as do not, and there's nothing wrong with doing your tank setup as a "berlin" (sand free) or a "monaco" (plenum/DSB ).
I followed Jean Jauberts methods for setting up my plenums right out of John H. Tullock's book "Natural Reef Aquariums." It's a fantastic book for beginner and advanced reefer alike. Here's what John & Dr. Jaubert recommend for plenum tanks:
Create the plenum plate out of a UGF plate or eggcrate. Cap off the UGF with the hole caps if need be. Place the plate in the bottom of the aquarium and cover it with a sheet of fiberglass or nylon windowscreening (non-metal) from a hardware store. You'll have to pre-cut the screen with sharp scissors as it comes in rolls like christmas paper.
Then, add a fine layer of non-calcarious/non-silica sand, such as quartzite sand or pool filter sand, to about 1.5-2" of uniform depth. This fine sand layer is your dentrification zone. The rationale behind a non calcarious sand for this layer is to avoid concretion problems commonly associated with silica and aragonite based sand. Experienced reef keepers have experienced this, when silica begins to stick to the sand and it tends to clump up like scoopable cat litter. You can break apart these clumps pretty easily by hand or with a pair of reef tongs, but it's going to be impossible to do this in a DSB. So, he advocates using a fine grade of sand as outlined above for this layer, to avoid concretion in the dentrification zone. Also, since aragonitic sand will dissolve over time, you don't want your bottom sand layer to be one you have to replace. You can replace the top sand layers every six months to a year or so by just adding new sand on top, but not the bottom layer.
After you've poured and leveled your first layer of sand, cover it with another layer of windowscreening. Then, add an additional 2-3" layer of aragonitic sand, such as caribsea seafloor special grade or araga-max. This layer should be variably rubbly/chunky with medium sized sand grains.
That's it. That's all there is to it. If the sand is too deep, it could turn anoxic and start to produce hydrogen sulphide gas. This is easily spotted as the tank will begin to stink like rotten eggs. You can remove 1" of surface sand and fix it. If the sand is too shallow, it might never develop a dentrification zone (low oxygen zone) and your tank will consistently show nitrates. The fix is to add a little more sand to the top layers.
Plenums take time to break in, just like cycling a new tank, before nitrate levels will fall. The more friable the upper surface layers are, the better, so a healthy worm/pod population really helps. Sand stirrers, such as nerite snails, white brittlestars and gobies will also help keep the sand from compacting as well.
I took my substrate one step further and seeded it with GARF Grunge+ (www.garf.org
), which is a mix of miracle mud, live sand and reef rock rubble. It's chock full of life and will rapidly seed aragonite sand. I love this stuff. I have all kinds of shrimps, pods, tubeworms, spaghetti worms and even teeny brittlestars crawling around in my sand thanks to Garf. Looking at the upper aragonite layer reminds me of looking at an ant farm as a kid, except that now i'm watching pods and little baby cleaner shrimps (they pink up early!) crawl through the substrate. Ooh, what is that.. what's that thing... where's my magnifying glass gone?
Is it the right way? Who knows. It's working very well for my tanks. All nitrogenous readings are zero, pH fluxuates a little between the light/dark cycle between 8.3 and 8.0 (I blame my DOC algaes) in one of my tanks because it has no refugium (the other does, and the pH is more stable). The alkalinity of the tanks run about 11 dKH and the calcium levels are approx 420 ppm.
I like the natural reef appearance and the benefits of NNR (natural nitrate reduction) the DSB/Plenum provides, as well as a further buffer for the pH and the reintroduction of calcium/strontium/magnesium as the aragonite layer dissolves slowly over time.
But that's just me!