'Filters For Dummies' Part 2 by Grant Gussie
Grant Gussie describes the different types of filtration. Copyright Calgary Aquarium Society 1998.
Introduction Continued
Carbon filtration requires the regular purchase of fresh filter media because the old carbon must be discarded when it is exhausted (carbon can not be economically reactivated at home). There are a large number of brands of activated carbons available. These carbons come from a variety of different sources, including wood, coal, and coconut shells. Most commonly used are bituminous and lignite coal.
Tests reveal that the most expensive carbon is not necessarily the best at removing organic matter from aquarium water. Surprisingly, the inexpensive Aquarium Pharmaceuticals Super Activated Carbon brand scored best of those brands commonly sold in local stores (Harker, 1998).
Carbon filtration is quite important in salt-water aquaria because marine fishes are very sensitive to pollution, and because of the expensive sea salt they require. It is therefore very expensive to make the massive water changes required to keep pollution levels low enough for salt-water fish. So it is in everyone’s best interest to purify the old water through carbon filtration. A lot of debate goes on as to how much carbon is needed, but current thinking is that you should use only a little carbon but change it often. Harker recommends about 1mL carbon per 4 liters of water (3 tablespoons per 50 gallons), and to use the carbon only intermittently, about 12 hours per week, using fresh carbon each week. Put the carbon in the filter so water flows through it, but only slowly.
Also recommended for marine aquaria is chemical filtration with a foam fractionator, also known as a protein skimmer. A large protein skimmer is now generally considered to be mandatory equipment for all salt-water tanks.
Skimmers work like this: fine air bubbles are injected into a downward flowing column of water. Hydrophobic molecules like lipids and most proteins adhere to the bubbles and rise with them to form a froth at the surface of the water column. This froth is pushed upward through an outlet by more froth that continues to collect below it. The froth then flows into a collecting cup for removal. By removing the organic molecules before they break down, the skimmer helps maintain a much higher water quality than can be easily maintained without one.
Skimmers inject air into the water by one of two ways. Air can be injected conventionally with an air pump and a fine air diffuser. This design tends to be used in lower capacity skimmers suitable for tanks smaller than 400 liters. But larger aquaria and reef aquaria usually employ venturi skimmers. These have powerful water pumps that force water through a narrowed opening into which an air inlet leads. This opening is called a venturi. The water must speed up to go through the narrowed opening of the venturi, and the faster a fluid travels the lower its pressure is. So air is drawn through the inlet to the inside of the venturi.
But alas, only very large, powerful, and expensive skimmers work in fresh water. This is because fresh water has a higher surface tension than salt water and so the air bubbles are larger – too large to cause the water to froth. Very energetic pumps are consequently required to froth fresh water, making a fresh water skimmer economically unviable. They consequently are not used in fresh water aquaria, but they are used in sewage treatment plants where pollution levels are much higher and frothing is more easily achieved.
So how do you chemically filter a fresh water aquarium? If a fresh water aquarium is full of very inexpensive tap water then the simple answer is…you don’t. Why bother? Tap water is cheap and so why not just change the water with fresh instead of trying to purify the old stuff? The nitrates, phosphates, and organic matter dissolved in your tank are great fertilizers, and since you have to water your lawn and houseplants anyway, just water them with old tank water and give your fish new water. While you are at it, rinse your mechanical filter foam in the wastewater before giving it to the plants so they get some solid fertilizer too. Nothing beats organic!
RECOMMENDATIONS
So, that should be about all you need to know about the various kinds of filters used in both fresh and salt water tanks. But for your first fresh water aquarium, I would recommend a hanging biological power filter and a lot of plants in the tank. For a fish room, I would recommend a central filter system using a combination wet/dry trickle filter and plant filter. For marine fish tanks, the same recommendations apply except that you should also add a carbon filter and a large protein skimmer. Marine coral reef tanks (which receive very little feeding) are better off with only a large skimmer and carbon filtration (no wet-dry filter), along with a large quantity of porous "live" rock that serves as an in-tank biological filter. But the filtration of coral reef tanks is a specialized topic beyond the scope of this article, so I would recommend you do some further reading before you attempt it.
REFERENCES
Hovanec, T. A. and E. F. DeLong. 1996. Comparative analysis of nitrifying bacteria associated with freshwater and marine aquaria. Appl Environ Microbiol 62:2888-2896
Harker, R. 1998. Granular Activated Carbon In The Reef Tank: Fact, Folklore, And Its Effectiveness In Removing Gelbstoff — Part One Aquarium Frontiers on Line May & June 1998?
Diatomaceous Earth Filtration
Also used as a mechanical filter medium is something called "diatomaceous earth". This is made of the hard silica exoskeletons of single-celled marine organisms called diatoms. These skeletons are not only very small, they are festooned with a baffling assortment of spikes, holes, hooks, and assorted appendages that trap suspended particles very well. Anything larger than a bacterium gets trapped by these filters, so they make the water very clear. That’s why they are often used in swimming pools.
Diatomaceous earth requires its own specially designed filter unit. These filters are tricky to set up because you have to suck up free-floating diatoms so that they coat the inside of a nylon filter bag before filtering your tank. You then run tank water through the bag until it clogs up. The bag is then rinsed out, discarding the diatoms along with the trapped gunk. The process is then repeated with a fresh coating of diatoms. Fresh diatoms must be purchased separately.
Because DE is so effective at filtering out anything and everything, these filters clog up in no time. And because of their short running time between cleanings, diatomaceous earth filters are only good for intermittent use. The idea is that you would only filter a tank for ½ an hour or so once a week, rather than running the filter continuously.
These filters were once very popular – aquarium magazines from the 60’s and 70’s were full of their ads – but nowadays, few people bother to use them because quite frankly they are more trouble than they are worth. Water clarity is more easily achieved through conventional filters and proper tank maintenance.
Denitrifiers
For a short time in the early to mid 90’s there were advertised a lot of different kinds of denitriying filters. But you don’t see them much anymore because they don’t work. That sort of put a damper on their sales.
Anyway, this is how they are supposed to work. Nitrate can be used by anaerobic bacteria as an oxidizer instead of oxygen. In effect, the bacteria breathe nitrate instead of oxygen. So if you create a region where there is no oxygen, but lots of nitrate, and a food source for the bacteria in the form of a carbohydrate, then the bacteria will consume the nitrate. The end result is nitrous oxide or nitrogen gas, which diffuses out of the water into air.
In denitrifying filters water is very slowly passed through a submerged filter medium. A small amount of a sugar (bacteria-food) is added. Aerobic bacteria consume all the oxygen in the outer layers of the filter medium, and so bacteria in the inner filter consume the nitrate in order to metabolize the remaining sugar. By the time the water passes through the filter, it has had all of the sugar and oxygen consumed, as well as at least some of the nitrate. The water is then aerated to replace the lost oxygen and get rid of the excess nitrogen gas. It is then returned to the tank.
So, why don’t they work? Well, in practice, they are just too touchy. Too much organic matter in the water and you get poisonous hydrogen sulfide being produced. Pass the water through too quickly and ammonia is produced. Etc., etc. And even if you do get them set right, you must pass the water through them so slowly that they do not produce enough nitrate-free water to make any real difference to your tank, unless you get a very big denitrification filter. That means big bucks.
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