Reefkeeping 101 -
Natural Filtration - Part 2
A Bedtime Story

Still Rockin’

At the same time that last month’s column was released there was a thread in the New to the Hobby Forum talking about curing live rock. It was a lively exchange and various views were presented. One of the threads questioned the need for water changes and using separate curing vats. The objection being that it cost far too much money for equipment and salt mix.

Actually, the cost of curing rock by that method is usually cheaper. Why? Well, when live rock is cured in say, a hundred gallon tank, you have a lot of water to make for not all that much rock. When using a curing vat, you can easily place about 50 pounds in a shallow 10-gallon tray. Now, do a total water change and only throw out about three or four gallons of water per tray, rather than 80 or 90 gallons, as there would be in a full tank. That, right there, is a big savings. True, ammonia levels will be far higher in the small vessels, but during the curing process ammonia levels will top 10 ppm in just about any size tank, which is toxic to all but the hardiest of species anyway.

The equipment is not that expensive and need only be as elaborate as you desire. If you're not having a living soul visit you for a couple of weeks, except possibly someone from the coroner's office who notices the smell, then just purchase a few inexpensive trays and do it in the living room. If one wishes to do it at a more remote location, some heaters or those warming mats I mentioned last month, are the only extra items needed. Heck, you can sell them to a new reefkeeper, once you are done with them. I had a more elaborate system for a while and also used a couple of shop lights and daylight bulbs to illuminate the vats. This helps kick-start the coralline re-growth on the rock.

Unless the rock is fully cured when it is purchased, and added to the tank within a day of purchase, then there will always be an algae outbreak. It occurs in two parts, the first, a brown algae outbreak caused by diatoms. Diatoms are algae that have a silica shell on the outside. Diatoms require soluble silica to survive, and they probably get it from dead, encrusting sponges on the live rock that decompose during the curing process. Usually, the outbreak only lasts a week or so. The silica is then depleted and the diatoms decline. Once this happens, the green algae take over and that outbreak lasts far longer. Adding snails at this point can help, as can frequent water changes. Do not add any algaecide products to your tank. Don’t forget that coralline is a beneficial alga, too. Eventually, the green alga goes away with its duration determined by the amount of nutrients that entered the tank during the curing process.

Here is an idea I’d like to share that I picked up from a friend who uses an illuminated curing line. In curing live rock I always waited until the ammonia level reached zero, remained there for about three days, and then would move the rock to the main tank. Following the move, however, I did have an algae outbreak that lasted at least three weeks to a month. It was not as bad, nor did it last as long as one gets when curing the rock directly in the tank, but it was still a pain. My friend goes a step further. He gets the outbreak going in the curing vats. Every couple of days he plucks out as much algae as possible and changes the water. After about two weeks the algae is nearly gone and he then moves it to his main tank. I really like the idea and, if a person has some shop lights or the vats are located to receive good sun most of the day, it is worth a try.

There was also some discussion about just using base rock and seeding it with a little live rock. The argument here was that bacteria will grow very fast and the rock will have a good bacteria covering in just a few weeks. This is certainly true, but live rock is far more complex than that. It has a diverse population with much more living on the rock than just bacteria and a little coralline. Higher organisms are also part of the rock’s “ecosystem” and help in the filtration process. These take far longer, if at all, to develop and are a key player not found on do-it-yourself cultured live rock. Remember, we are trying to recreate natural filtration in the same manner that it functions on a true tropical reef. That means having live rock that has the type of diversity that occurs in nature.

This even applies to aquacultured rock. It’s best to have rock that is cultured directly in the ocean or at least has a constant supply of fresh ocean water circulating through the curing tanks. It should also be properly aged, meaning that it is cultured for at least six months or, even better, for a year or more. Just like its natural harvested relatives, things like feather dusters and encrusting sponge on the rock indicate proper culturing has been practiced. If you can get rock of that quality, then always ship it next day air. The extra price for shipping it that way will pay off in that the rock will need almost no curing period and those nasty algae outbreaks will be minimal.

Getting to the Bottom of Things

Enough on rock for the moment; let’s talk sand. One of the most debated issues on Reef Central revolves around having substrate on the bottom of the tank. Should one have any? If so, what type? How much should one use? These are all debatable questions. I will tell you up front, I am still an advocate of using a deep sand bed (DSB), having sand that is primarily aragonite, supplemented with as much true live sand (LS) as one’s Stimulus Payment Check can buy. To me, it is still the method of choice for most reefkeepers and especially for the novice reefkeeper. True, not having a sand bed can save a lot of money, but having one is money well spent - at least in my opinion.

At the forefront of the DSB movement has been Dr. Ron Shimek. Dr. Ron has set down the fundamentals for live sand, namely:
    The bed should be at least four inches deep.
    The sand should be quite fine, sugar-sized, in the range of .05- 2 mm.
    The bed should not be disturbed.
    Worms and other sand-shifting organisms should be present to stir the bed.

Beds need to be deep enough so that zones are created in the bed. The upper levels have decreasing oxygen tension, and organic compounds are processed by aerobic bacteria. Lower in the bed, oxygen approaches near depletion and bacteria use the oxygen contained in the nitrates, produced at the upper levels, to further degrade organics but using the oxygen from the nitrate molecule. This anoxic zone is where denitrification occurs and is the main reason for having a DSB in the first place. Four inches is the suggested depth, but having slightly less usually doesn’t hurt. Extremely deep beds, on the other had, run the danger of becoming fully anaerobic at their lower levels. That is not an ideal condition and should be avoided. Even at 4-6” there are some anaerobic areas in any sand bed, but those over 10” are more likely to have substantial zones. This invites conditions where sulfates can be reduced to toxic hydrogen sulfide.

One of the necessary conditions for a bed to be successful is that it has a continuous turnover caused by sand-shifting organisms in the bed. This means that sand should be fine enough for these organisms to move about. Very fine sand can actually be moved around by the combined efforts of the bacteria in the bio-film on the sand. The other reason for fine grains is that they provide a far larger surface area than large grains, thereby supporting greater organic loading. Having sugar-fine sand or finer is therefore far better than having something, like crushed coral, for the bed.

As I explained above, it is desirable to have zones of decreasing oxygen tension existing in the bed. It takes time for these zones to develop, so we want to keep them intact. Adding large sea cucumbers, large burrowing fish or big starfish, which can all disturb the bed, should be avoided. So should using one of those sand vacuums, so popular for cleaning substrate in a freshwater tank; they should be avoided unless it is just used to clean the very top of the bed.

While it’s not a good idea to have large burrowing creatures in the bed, we do need the tiny ones. Various worms, mini starfish, snails, copepods, burrowing shrimp and tiny sea cucumbers are all welcome members of the sand’s living community. They are very important, and without them the bed may go completely anaerobic in time. If these basic rules are followed, then we can have a bed that is productive over a long period of time. Like everything else in this hobby it does require some maintenance.

At this point we need to talk a little about getting a new sandbed started. The best, and fastest, method is to just buy enough live sand to get a four inch bed. When using this method I do suggest that you purchase sand from several suppliers. This provides better diversity in sand organisms than getting it all from a single source. When starting with new live sand there is really not a curing period involved. I merely dump the sand into a five-gallon bucket of saltwater. I do not mix it, but I decant off the water in the bucket before emptying it into the main tank. This step will flush out light dead organisms from the new sand. Do not vigorously stir the sand and never, ever place it in a fine strainer to rinse it. That will remove sand and valuable organisms from that sand that costs good money to obtain.

Since I use a vat curing method for the live rock, I get the live sand at the same time and place it in the main tank. I then use the lights and pumping equipment while it is in the main tank and check ammonia levels for the first couple of days. You may see some increase in the ammonia level, but nothing like what is seen with curing live rock. When the live rock is ready for the main tank, I do one massive water change and then add the rock. The advantage to this method is that efforts can be started to arrange base rock or other rock supports in the sand while waiting for the rock to cure.

A Little “Rockitecture”

I’ve done some new tank set-ups for others. I was, some years ago, gently placing live rock on the tank’s glass bottom so I could then add live sand. The dentist, whose tank this was, saw me doing this and yelled out, “Why are you burying that live rock I just paid over $7 a pound to buy?” Why indeed? My friend had made a very valid point. That part of the rock that was buried under 4” of sand did not serve much in the way of filtration. Since then, I always use base rock or some other type of support to mount the rockwork near the surface of the sand.

Now base rock is just dead rock that is placed on the bottom of the tank and stacked to a level an inch or two above the top of the sand. If you don’t have base rock, although I have no idea why one wouldn’t, then nylon dowel material can be purchased at Lowe’s or Home Depot, cut to a length slightly shorter than the top of the bed and the live rock placed upon it. You don’t want the dowels to protrude above the sand bed or some enterprising sea critter will excavate the area where there are gaps between the dowels. Landslides are almost always bad events in a reef aquarium. I would suggest doweling that is about 1.5-2” in diameter for this purpose. Some people use PVC pipe to do the same as the dowels, but I’m somewhat against that as the PVC has a big hole in the center. This means there will be sand trapped in the hole. When it is capped off with live rock it will certainly go anaerobic, which is not desirable.

I’m starting to feel like “Ask Heloise” with her “Thousand Uses for Nylon Net” but smaller dowels are great for live rock landscaping. Do you need ledges and shelves in your rock arrangement? It’s easily done using 1/8 to ¼ inch nylon dowels. Drill holes in the pieces to be jointed using a masonry bit, a 32nd larger than the dowel. Then, using super glue gel, glue both ends of the dowel and place in the two rocks to be jointed. Silicone can also be used, but it does not stick very well to the nylon rod. For larger ledges this may not be enough support. Here, those plastic “L” bookshelf holders can be used for more support. Don’t worry that you can see them at first, as they soon become covered with coralline. Be careful when joining rockwork together. It is no fun when the weight of that beautiful creation that was glued together is too heavy to lift into the tank. Do it in manageable sizes and weights.

Arranging rock is beyond the scope of this article and, it so happens, that many feel it is beyond the scope of yours truly, who is not much of an artist. Just do it to your liking or look at any of Reefkeeping’s Tanks of the Month for ideas. I do suggest that if you want clean glass on all sides of your tank, don’t place the live rock against, or so close to, the tank’s walls that a scrapper cannot be used to remove the algae.

Mr. Sandman, Bring Me a Dream

When using all live sand there is no problem setting up the tank, well, except the cost. Many reefkeepers feel it is a little too steep. The usual solution has been to use base sand (dead sand) and mix it with live sand. This method does work pretty well and, in years gone by, there was Southdown Tropical Play Sand to the rescue. Why Southdown? Well, true live sand is actually a mineral called aragonite. It is composed of calcium carbonate composed mainly of shells and skeletons of dying reef creatures over hundreds or thousands of years. The advantage of aragonite is that it not very dense (specific gravity ~2.94), lacks sharp edges and has buffering capability (can neutralize acids). Since it very abundant and natural to the reef environment, it is perfect as base sand. That was the wonder of Southdown; it was pure aragonite and available at the hardware store for under $10 for a 50 pound bag. It was perfect - and then they stopped marketing it. It is so rare today that threads continually pop up citing appearances that, like UFO’s, are never confirmed.

You can still buy aragonite sand but usually only at the local fish store (LFS). It does cost a lot more than Southdown, but it is still true aragonite. “Live sand” in bags, also sold at the LFS, is something to avoid, however. It is just a bag of dead aragonite, with some nutrient solution added, and a few cultured bacteria. It’s artificial from the word “GO” and lacks all those tiny, sand-shifting organisms so important to a great deep sand bed. It is rarely worth the extra money.

The question arises, “If cheap aragonite is not available, can other sands be used?” The answer is yes. While not as perfect for your tank as true aragonite, they too can be used as base sand. First off, it must have the proper size, in the range of 0.05-2.0 mm. The majority of the particles should fall in the range of 0.15 through 0.25 mm; about the same size as fine-grained, granular sugar. Some, not all, silica sands can be found that meet these criteria. Usually that will be found in silica play sand or sand box sand. The major drawback is they are denser and have relatively sharp edges. They also offer no buffering ability at all.

Let me talk a bit about buffering. Once oxygen is depleted in the bed, anaerobic organisms appear. These guys process the food they consume mainly by using fermentation - “Cheers!” Yes, they do make alcohol but carry it out too far. You end up with organic acids, like vinegar, a completed fermentation by-product. Now, pour some vinegar over aragonite and it fizzes like an Alka-Seltzer. This is because the acid, vinegar, reacts with the base, calcium carbonate, and produces a neutral salt - calcium acetate. The net result is that some sand dissolves but the acid is neutralized.

There is a side effect, however. The reaction also produces free carbon dioxide that, dissolved in water, produces a weak acid, carbonic. In a perfect world this would reduce the bed’s pH below 6.5 where the carbonic acid reacts with the calcium carbonate in the aragonite forming calcium bicarbonate; a very good thing as it adds calcium into the water column.

Alas, it is not a perfect world. In the days of yore it was claimed that having a deep sand bed was the way to go as it provided all the calcium a tank would need. Sadly, this is not the case and having a deep sand bed is not the road to having a calcium-dominated tank. Only a small extent of the sand bed meets these conditions and the amount of calcium dissolved will nowhere meet the calcium requirements of most hobbyists’ tank. Yet, I’m always the optimist and, even though small in its contributions, every bit helps.

Notwithstanding, silica sand does work. It was once thought that silica sand would release untold amounts of silicates into the water column, thereby fostering a continual diatom outbreak. Sand is a major component of the glass in your tank, and if it dissolved, be prepared for a flood even Noah can’t endure! Silica sand is inert.

Casting the Seeds of Life

When any form of base sand is used, some seeding will be required. Some people just let the live rock in the tank seed the sand bed. Not only is that slow, it only seeds it with bacteria and very few other organisms. Such a bed does not contain the sand-shifting organisms so important to bed health. Almost no water circulation occurs in such a bed, and it quickly becomes stagnant. Some purchase detrivore kits that provide worms and such, but they are expensive and may cost as much as using real live sand does in the first place, especially in smaller tanks. Others get live sand from a buddy and use this. That is better, but in a large bed it will take more than most “buddies” will want to donate.

The best way is to buy some real live sand. I suggest at least 25% of the bed be true live sand, but sometimes less is used. I recommend that live sand be obtained from several suppliers. It does add to the shipping costs but provides better diversity, an important aspect in bed health. When using this method the base sand is added first then the live sand is placed on top. This avoids crushing critters living in the live sand. Then, using a nonmetallic pointed object (a pencil will do), some of the live sand is pushed into the base sand. This helps zones in the bed to form faster.

When using base sand there is always a sandstorm that my take a week or so to settle. Some people wash the base sand first, but those fines actually help the bed. It is best to just wait it out. These sandstorms will arise at first in a new tank without too much provocation. Be assured that they abate in occurrence as the bed matures. This is because the bacteria in the bed secrete glue-like substances to hold onto the sand grains. Over time, they also cause smaller particles to stick together, thus quelling the storms.

Once the bed is in place, and the cured live rock arranged, the tank is close to being ready to stock. There is some maintenance involved in maintaining the bed, but it is only periodic. As time passes, through predation and natural die-off, the sand-shifters decline in numbers. When that happens one must add new seed to the bed. In an aragonite bed there is some dissolution of the bed over time, and adding new sand is just a matter of topping things off. With silica sands you must remove some before adding fresh sand. Always do this using sand near the surface and avoid stirring the lower part of the bed. I like to add seed annually and usually add about 5 lbs. of seeding live sand per 100 lbs. of bed. Not a lot, but it provides the needed sand stirrers. Having a refugium with live sand somewhat decreases the need for this seeding as it provides a safe haven for the sand-shifters.

Well, we are not into refugiums yet, and it is probably time to close for this month. Next month we will explore other options for providing natural filtration. Some are pretty cool, but others are old hat that has been passed over.

Off to bed with you...

Additional Reading:

How Sandbeds Really Work by Dr. Ron Shimek

Please note - I use the environmental chemist's definition of anoxic - an area of low (<0.5ppm) oxygen tension, and anaerobic - devoid of all oxygen. Doc Ron uses the biologist's definition that reverses those two terms. Otherwise the science is the same. - Tom

If you have any questions or comments about this article, please visit this thread in the New to the Hobby forum on Reef Central.