3.1.1 TRADITIONAL BIOLOGICAL FILTRATION METHODS:

By traditional filtration methods we refer, in this case, to the undergravel biological filtration method, a method that is still used by thousands upon thousands of hobbyists and has "proven" itself over the years. Just about every book on aquarium keeping you read, advocates this particular type of filtration. Spotte (1979) has written extensively, if not exhaustively, on the subject, and so have many other others.

Undergravel filtration, whether reverse flow or normal flow, obviously works very well. There is no question about that and there never has been in my mind. Our hobby would not be where it is now it weren't for that filtration method. The point that has to be made, however, is that in reef tanks a still better type of filtration can be used, and should be used: wet-dry filters, also called trickle filtration. We shall discuss this method of filtration in great detail later in this book, as the water quality parameters obtained when using such filters is by far superior to the ones found when using any other form of filter, including undergravel filter plates and canisters.

Undergravel filtration can, of course, be used in conjunction with many other types of biological filtration. Often hobbyists supplement U.G. filters with Eheim-type canister filters, outside box filters, and also with Lifeguard or PEP-type canister filters. Such is, of course, all to the benefit of the system on which these filters are used, leads to better water quality, and better looking aquariums. Notwithstanding all such filtration, and the excellent levels to which the water chemistry can be brought, it is still possible to improve on those methods. Again, wet-dry or trickle filters are the answer, especially when they are combined with adequate chemical and mechanical filtration means.

Undergravel filters require a lot of maintenance, especially to prevent the detritus and dirt that accumulates in them from becoming a problem. If you are of the opinion that your undergravel filter does not trap dirt, we suggest that you stir up the gravel a little, and see for yourself what we mean.

Although some authors suggest that a well maintained undergravel filter will not trap dirt and will not clog, we have never found that to be the case for any length of time. Because the water that is pushed through, or sucked into, the gravel always contains impurities, eventually certain areas clog. As the clogging increases, certain areas become low in oxygen content and may turn anaerobic. This leads to a lowering of the water quality on one hand, and possibly the appearance of hydrogen sulfide on the other. The latter is a gas that mixes very readily with the water and can result in both fish and invertebrate losses.

Even water that is pre-filtered results in the undergravel filter eventually clogging, the only difference being that it will take longer for such to occur, but clog it will. Not only is clogging a problem, but the flow through an undergravel filter itself can be problematic as well. Indeed, the flow is uneven and greater where the water finds less resistance, for example around the base of rocks or pieces of coral placed on the gravel, mainly because the layer of gravel is much thinner in such places. Such does not promote efficient biological filtration.

To prevent the formation of anaerobics, and to reestablish a better flow through the gravel, hobbyists clean the U.G. filters from time to time. This is fine, insomuch as doing so increases the efficiency of the filter, but hobbyists must realize that such happens at the detriment of the water quality, for a few days to a week, each time the filter is cleaned. Indeed, cleaning the U.G. filter destroys a great number of the bacteria that had settled on the gravel kernels, which destabilizes the filter until those bacteria have repopulated the gravel. Less bacteria provide less as well as less efficient filtration, and the latter will result in stress on the fish and invertebrates.

Such is certainly not conducive to having a vibrant looking tank. Even when only portions of the filter are cleaned at a time, bacteria are still destroyed and the above scenario is still accurate. Additionally, how undergravel filters are run greatly influences their efficiency, for example, some parameters to look at include: the amount of water flowing through the gravel bed (usually dolomite or crushed coral), the thickness of the gravel bed, the size of the kernels, etc. All influence the amount of filtration that actually occurs and the quality of the resulting water. This in turn determines how well the fish and invertebrates in the aquarium will do, how they look (appear), and whether or not they will survive for any length of time in your tank.

Marine Aquariums, and especially marine "reef" aquariums that look at their best, will always show high levels of dissolved oxygen in the water on one hand, and very low levels of pollutants such as nitrates, NO3, and phosphates PO4, ammonia, NH3, and nitrite, NO2 on the other.

Keeping dissolved oxygen levels high is not all that easy to do in tanks run with undergravel filters, not because the hobbyist does not take the time and care to achieve such levels, but mainly because such filters are not as conducive as others to such high levels of oxygen, O2, in the water. It has nothing to do with hobbyists' maintenance practices, but with the nature of the filter itself.

Some hobbyists resort to a method called "reverse" flow undergravel filtration, as opposed to flow through from top to bottom. Reverse flow is a system whereby the water is pushed through the gravel from bottom to top (the reverse of what is normally done). Many hobbyists report excellent results with this form of filtration, better, in fact, than with the regular type of undergravel filtration. What is very important when using this approach, is to fine filter (mechanically filter) the water extremely well, to prevent the material used on top of the filter plates that make up the undergravel filter, from trapping dirt, and eventually clogging up.

How such a system is set up is explained in great detail in some of the other aquarium books that you may have read, or in Eheim's instruction manual, the one that comes with the special equipment they sell to install such a form of filtration.

Many hobbyists too will want to run their aquariums, including reef tanks, while keeping the undergravel filters that are already in place, supplemented with, perhaps, a trickle filter. Such is fine of course. The set up can, however, be greatly improved upon by removing the undergravel filters altogether. How to do so, and why it is a better way to run the tank, is explained later in this book.

If it is not practical to remove the undergravel, or if doing so requires that the entire tank be taken down and started up all over, then it may make sense to keep the filter in place and add supplemental means of filtration to the existing arrangement, thus improving the overall efficiency and water quality. I am certainly not suggesting that you must remove undergravels to be able to run a decent reef tank. What I am saying, is that you will have better results when you do, and use the even more efficient trickle filtration instead.

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