Sand: DSB vs. Plenum

As an advocate for plenum sand setups, Bob Goemans is taking a lot of punches these days. People in this hobby are a peculiar lot and passions flare up often. Not so much about the plight of the Red Sea or destructive fish collection methods, but about what they perceive as a right or wrong way to setup a reef tank. The sand bed in a reef tank is often the epicenter of such debates.

Going from word of mouth and Internet trends, it appears more people are adapting the DSB (Deep Sand Bed) method. Many are following in step behind Ron Shimek, a Marine Ecologist/Invertebrate Zoologist and an advocate for DSB. Dr. Ron has not been above throwing some pot-shots at Dr. Bob in some of his posts out there on the Internet - *I have read some that go below the belt!

Likewise, others that speak out in favor of plenums, are often met with such reactions elsewhere. The irony is that, the more I dig, the more it appears both approaches can be equally successful in reducing nitrates. So, to quote Mic Jagger at Altamont - "Who's fighting? What for?"

There are other methods too, though not as popular in the USA, such as barebottom tanks and tanks with very little sand too. Some benefits of these are the ability to have a higher water flow rate, no dependence on 'bugs' or worms and more room. However, they do require a more hands-on approach with more frequent siphoning and water changes.

Both plenums and DSBs have around 4 inches of sand, but the main distinction is the plenum method utilizes a foundation of pvc/egg crate on which the sand is placed to aid in denitrification.

Dr. Jean Jaubert was one of the first to develop a successful live sand filter for the aquarium. His systems have been widely adapted and are very successful at keeping the nitrate-nitrogen level at below 2 ppm.

The basic Jaubert setup consists of three layers (from bottom to top):

The plenum area

The coarse sand layer (5-6mm) 1" deep

The fine sand layer (2-3mm) 2-3" deep

This is really the 'key' to this method of sand filtration. The plenum area is a motionless open layer at the bottom of the tank that acts as a 'chemical sink' or anoxic zone. This layer accumulates a small amount of oxygen along with nitrate that diffuses downward though the sand. The anoxic conditions of the plenum area and the layer above prevent anaerobic conditions in these areas while promoting denitrification in the lower sand layer.

The plenum layer should be about 1 1/2" high in aquariums under 100 gallons. In aquariums between 100-500 gallons, it is recommended that the plenum be 1 1/2" to 3" in height. The most common setup is pvc pipe on its side supporting egg-crate above it. The pipes have holes drilled in them to aid in diffusion. Make sure that the plenum is not exposed to light on the sides - you can leave a small gap around the edges and fill with sand. This way the plenum will not be seen at all. A common plastic window screening is usually placed over the egg-crate to prevent the coarse sand from dropping into the plenum area. Make sure to cover every side well to prevent this from happening. I folded the screening like a Christmas present and lit the edges with a lighter and smeared with my finger to get a good seal. What a pyro. In hindsight, silicone would have been a little safer. Heh.

--> Check out Garf's Plenum Calculator!

Bob believes the jury is out on the benefits of having a coarse and fine sand layer. He has adapted a single sand layer approach like that of DSB. He recommends one layer, 4 inches deep, of either aragonite or *crushed coral that is 1 to 4mm in grain size.

He firmly believes that the plenum provides the more efficient form of dentrification because it has a little bit of oxygen (0.5-2.0 mg/l) in the sand layer creating an anoxic zone. This zone helps convert nitrates back to their gaseous state. Where as a DSB with the sand placed directly on the bottom of the tank creates an anaerobic zone which converts nitrates back to ammonium and no further. This is seen as a shortcoming of DSB because the ammonium is a preferred algae nutrient.

Source: F.A.M.A.

For more up-to-date details, pick up an issue of F.A.M.A. -- 'Freshwater And Marine Aquarium' magazine and read the 'sand-mail' column. Here is a picture he sent me of his 125 gallon tank.

(* Some people are not big fans of crushed coral and refer to it as a trap for debris.)

DSB is probably the more popular of the two substrate methods commonly used today. The deep sand bed creates an anaerobic zone to break down the nitrates.

DSB also utilizes sand from 3-6 inches in depth and Dr. Shimek suggests a grain size of .05 to .2 mm (average .125mm).

To save money, many people go to Home Depot and buy Southdown Sand.

DSB is easier than a plenum because it does not require a pvc/eggcrate foundation. You merely add the sand to the bottom of the tank. Supplemented with a good clean up crew of crabs, stars, snails, worms, etc.. this substate will work well. It is also more appealing to some because it takes less room in the tank than the plenum method.

Many people jumpstart the whole process by purchasing 'live' sand. Live sand already contains the necessary bacteria, pods, etc.. If you can afford it, great! If not, another jumpstart alternative is to simply seeding your sand bed with some sand from another established tank. But be careful. Know the history of the tank you get it from. Be wary of tanks that have been medicated, used tap water, etc. This can be more trouble than it is worth. This method is quicker, but not necessary. A normal sand bed will become active with time.

DSB, in theory, relies more heavily on detritivores than the plenum method. Many restock critters in their sand from time to time.

Detritivores / Critters

Many of the following critters are added to the sand to aid in denitrification:

Bristle Worms

Conches

Cukes

Scarlet Crabs

Nassarius Snails

Brittle Stars

For more info and good package deals, check out this sites:

Indo-Pacific Sea Farms

Inland Aquatics

eTropicals.com

Dr. Ron Shimek's Critter Page

So where do you go from here? I really think that these two methods in practice run a close race. Others will say differently, but it's my page, my opinion, right? Just like it is your tank and your choice. Get the right kind of sand and do what you want.

ALL sand beds require periodic grooming even with a good clean up crew. If you have the really fine sand like Southdown, use a thick straw, chopstick, etc., to break up the sand now and then. If you have crud collecting on it, you CAN use a tall siphon vacuum in sand to hit trouble spots. Use your thumb over the end the hose to control the siphon and prevent the sand from being sucked out of the tank. Don't go overboard and do every inch of the sand bed. You may do more harm than good.