Before a
discussion of biological filtration can happen, we need to discuss the
nitrogen cycle. In very basic terms, the nitrogen cycle is the biological
method of getting fish pee out of the pool before anyone becomes sick.
The
Nitrogen Cycle
Whether eaten
or not, natural process render most food into ammonia, which can be
toxic in concentration. Bacterial processes in the tank will convert
this toxic ammonia into equally toxic nitrite. Wow, what a deal. Now
other beneficial bacteria convert the toxic nitrite to nontoxic nitrate.
Actually, in significant concentrations of nitrate can be toxic, or
at best have a negative impact on water quality. This "cycle"
is the process that must take place in a marine tank, or your gonna
have a bunch o' dead stuff.
Now, every
new tank must be "cycled." This cycle is closely related to the
nitrogen cycle discussed above. But in this case, we are referring to
establishing significant concentrations of beneficial bacteria in order
to support the aforementioned nitrogen cycle.
This is how
the cycle is achieved: After setting up the tank, some hardy species
such as the much-maligned damsels are added to the tank and fed regularly
(you can actually use only live rock if you wish, as dying matter in
the rock produces the necessary ammonia source). In a few short days
the concentration of ammonia climbs steadily due to an inadequate concentration
of the right types of bacteria. The fish are a little tense by now;
clamped fins and rapid breathing. Over time, several more days, the
beneficial bacteria gain a foothold and the ammonia concentration starts
to drop. Now here comes the equally toxic nitrite. Since the necessary
bacteria for nitrite conversion still don't exist in concentration,
nitrite quickly ramps up to toxic levels. The little fishes are stressing
out by now; clamped fins, and pumping gills, and hiding in the corner.
Finally, the appropriate bacteria levels are present and nitrite starts
to decline, leaving nitrate in its wake. This is a good thing. Once
ammonia and nitrite have dropped to 0 and nitrate starts to rise, the
tank is cycled. Now you can start adding livestock…slowly. See
the animation below for a visual representation of the "cycle."
Now, back
to biological filtration. The beneficial bacteria necessary to support
the nitrogen cycle need someplace to grow. Traditionally this has been
accomplished using an undergravel filter and crushed coral. By pulling
water through the undergravel filter, using an airstone or powerhead,
bacterial colonization is encouraged in the huge amounts of surface
area available in the crushed coral. This was the best idea for a long
time, but it had a couple of drawbacks.
Pulling water
through this crushed coral also pulls detritus and junk into the filter
bed eventually clogging it and reducing its effectiveness. Also, due
to the endless supply of oxygenated water, aerobic bacteria was abundant,
yet anaerobic bacteria, that which converts nitrate to harmless free
nitrogen, was not. Therefore, nitrates had to be controlled with water
changes. Overtime, the use of undergravel filters fell out of favor.
(So did wet/dry filters due to their propensity to become "nitrate factories.")
What allowed
undergravel filters to be shunned was the widespread use of live rock.
Now the rock isn't really live. Maybe some things on it are, but the
rock is rock. Actually, it is usually coral rubble retrieved from the
reef. Highly porous, this limestone material makes the perfect realistic
aquarium decoration and an excellent habitat for aerobic bacteria. But
what about the nitrate?
Well, now
that there is no longer an undergravel filter on the bottom of the tank,
what can that area do for us now?. It just so happens that a ~4" deep
mixture of crushed coral in varying sizes and oolitic sand makes and
excellent tank floor covering and it helps reduce nitrate. Since no
longer are we pulling oxygen-rich water through the sand bed, the aerobic
bacteria is not so prevalent, and the anaerobic bacteria takes over.
It is this anaerobic bacteria that converts the nitrate to free nitrogen,
and it is released to the atmosphere. Now the nitrogen cycle is really
complete, and the goal of biological filtration is fulfilled.
A quick note
about the use of live sand as a tank bottom. Live sand is no more alive
than live rock is. What are alive are all the things in the sand; bristle
worms, mini starfish, and all kinds of thingies. You can purchase live
sand at a couple of bucks a pound, or in a sense you can grow your own.
A 4" bed of live sand for a 40-gallon breeder would require ~90lbs,
or more than $180. That's a lot of dough for sand. I use 80lbs of the
mixture of small crushed coral (called reef sand) and oolitic sand mentioned
above, and another 10lbs of commercially bought live sand spread on
top to seed the bed. Also, each time I visit a new aquarium store, I
buy an additional pound of live sand and add it back to the existing
bed. I feel that this greatly increases the bio-diversity of the sand
bed, but I could be wrong. (See The
Why's and How's of Sand Beds, with Ron Shimek at reefs.org.)
